clang: mm/khugepaged.c:1729:7: error: variable 'index' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]

[Naresh Kamboju <naresh.kamboju@linaro.org>]

Following build warning / errors noticed while building x86_64 with clang
on Linux next-20221025 tag.

Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>

## Build
* kernel: 6.1.0-rc2
* git: https://gitlab.com/Linaro/lkft/mirrors/next/linux-next
* git branch: master
* git commit: 89bf6e28373beef9577fa71f996a5f73a569617c
* git describe: next-20221025
* test details:
https://qa-reports.linaro.org/lkft/linux-next-master-sanity/build/next-20221025

## Test Regressions (compared to next-20221024)
Regressions found on x86_64:

 - clang-nightly-lkftconfig
 - clang-12-lkftconfig
 - clang-14-lkftconfig
 - clang-13-lkftconfig
 - clang-14-lkftconfig-kcsan

make --silent --keep-going --jobs=8
O=/home/tuxbuild/.cache/tuxmake/builds/1/build LLVM=1 LLVM_IAS=1
ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu- 'HOSTCC=sccache clang'
'CC=sccache clang'
mm/khugepaged.c:1729:7: error: variable 'index' is used uninitialized
whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
                if (!xas_nomem(&xas, GFP_KERNEL)) {
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:2063:47: note: uninitialized use occurs here
        trace_mm_khugepaged_collapse_file(mm, hpage, index, is_shmem,
                                                     ^~~~~
mm/khugepaged.c:1729:3: note: remove the 'if' if its condition is always false
                if (!xas_nomem(&xas, GFP_KERNEL)) {
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:1716:6: error: variable 'index' is used uninitialized
whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
        if (result != SCAN_SUCCEED)
            ^~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:2063:47: note: uninitialized use occurs here
        trace_mm_khugepaged_collapse_file(mm, hpage, index, is_shmem,
                                                     ^~~~~
mm/khugepaged.c:1716:2: note: remove the 'if' if its condition is always false
        if (result != SCAN_SUCCEED)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:1705:15: note: initialize the variable 'index' to
silence this warning
        pgoff_t index, end = start + HPAGE_PMD_NR;
                     ^
                      = 0
mm/khugepaged.c:1729:7: error: variable 'nr' is used uninitialized
whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
                if (!xas_nomem(&xas, GFP_KERNEL)) {
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:2064:15: note: uninitialized use occurs here
                addr, file, nr, result);
                            ^~
mm/khugepaged.c:1729:3: note: remove the 'if' if its condition is always false
                if (!xas_nomem(&xas, GFP_KERNEL)) {
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:1716:6: error: variable 'nr' is used uninitialized
whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
        if (result != SCAN_SUCCEED)
            ^~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:2064:15: note: uninitialized use occurs here
                addr, file, nr, result);
                            ^~
mm/khugepaged.c:1716:2: note: remove the 'if' if its condition is always false
        if (result != SCAN_SUCCEED)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:1710:8: note: initialize the variable 'nr' to silence
this warning
        int nr;
              ^
               = 0
4 errors generated.
make[3]: *** [scripts/Makefile.build:250: mm/khugepaged.o] Error 1

steps to reproduce:
# To install tuxmake on your system globally:
# sudo pip3 install -U tuxmake
#
# See https://docs.tuxmake.org/ for complete documentation.
# Original tuxmake command with fragments listed below.
# tuxmake --runtime podman --target-arch x86_64 --toolchain clang-14
--kconfig defconfig --kconfig-add
https://raw.githubusercontent.com/Linaro/meta-lkft/kirkstone/meta/recipes-kernel/linux/files/lkft.config
--kconfig-add https://raw.githubusercontent.com/Linaro/meta-lkft/kirkstone/meta/recipes-kernel/linux/files/lkft-crypto.config
--kconfig-add https://raw.githubusercontent.com/Linaro/meta-lkft/kirkstone/meta/recipes-kernel/linux/files/distro-overrides.config
--kconfig-add https://raw.githubusercontent.com/Linaro/meta-lkft/kirkstone/meta/recipes-kernel/linux/files/systemd.config
--kconfig-add https://raw.githubusercontent.com/Linaro/meta-lkft/kirkstone/meta/recipes-kernel/linux/files/virtio.config
--kconfig-add CONFIG_IGB=y --kconfig-add
CONFIG_UNWINDER_FRAME_POINTER=y LLVM=1 LLVM_IAS=1


tuxmake --runtime podman --target-arch x86_64 --toolchain clang-14
--kconfig https://builds.tuxbuild.com/2Gc0JDbUOIN0veL39dqIt03qgEi/config
LLVM=1 LLVM_IAS=1

Build link,
 - https://builds.tuxbuild.com/2Gc0JDbUOIN0veL39dqIt03qgEi/

--
Linaro LKFT
https://lkft.linaro.org