[PATCH v2] xfstest: add detection for ext4.h presence in configure.ac

[Disha Goel <disgoel@linux.ibm.com>]

In some distributions, __u64 is already defined in system header files,
causing compilation errors when building xfstest.

        # make
            [CC]    ext4_resize
        ext4_resize.c:17:28: error: conflicting types for '__u64'
         typedef unsigned long long __u64;
                                    ^~~~~
        In file included from /usr/include/asm/types.h:26:0,
                         from /usr/include/linux/types.h:5,
                         from /usr/include/linux/mount.h:4,
                         from /usr/include/sys/mount.h:32,
                         from ext4_resize.c:15:
        /usr/include/asm-generic/int-l64.h:30:23: note: previous declaration of '__u64' was here
         typedef unsigned long __u64;
                       ^~~~~

To address this issue, configure.ac now checks for the presence
and compilability of <linux/ext4.h>. If found and compilable, the
macro HAVE_LINUX_EXT4_H is defined. If not, a warning message
is displayed. The commit also updates src/ext4_resize.c to
conditionally include <linux/ext4.h> based on the presence of
the header, ensuring compatibility with systems where ext4.h
is either present or not. This change enhances the configure
process and improves code consistency.

The changes were tested on various distributions on Power
architecture, by successfully compiling xfstest. Additionally,
verified the compatibility by running ext4/033 and ext4/056
tests, both of which use ext4_resize and observed successful
test execution.

        # make
            [CC]    detached_mounts_propagation
            [CC]    ext4_resize
            [CC]    t_readdir_3

Signed-off-by: Disha Goel <disgoel@linux.ibm.com>
---

Changelog:
v1 -> v2
Drop usage of uint64_t to match the definition in the kernel uapi
headers. Use <linux/ext4.h> or <linux/types.h> based on the
presence of the header to get the __u64 definition as suggested
by Darrick.

Link to v1:
https://lore.kernel.org/fstests/20231221061231.44347-1-disgoel@linux.ibm.com/T/#u

 configure.ac      | 8 ++++++++
 src/ext4_resize.c | 6 +++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index b22fc52b..76378e6d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -35,6 +35,14 @@ AC_CHECK_HEADERS([	assert.h		\
 			sys/mman.h		\
 ])
 
+AC_COMPILE_IFELSE([
+    AC_LANG_PROGRAM([#include <linux/ext4.h>], [])
+], [
+    AC_DEFINE([HAVE_LINUX_EXT4_H], [1], [Define if ext4.h is present])
+], [
+    AC_MSG_WARN([ext4.h not found or not compilable.])
+])
+
 AC_CHECK_HEADERS([xfs/xfs_log_format.h],,,[
 #define _GNU_SOURCE
 #include <xfs/libxfs.h>])
diff --git a/src/ext4_resize.c b/src/ext4_resize.c
index 78b66432..51bd8d78 100644
--- a/src/ext4_resize.c
+++ b/src/ext4_resize.c
@@ -14,7 +14,11 @@
 #include <sys/ioctl.h>
 #include <sys/mount.h>
 
-typedef unsigned long long __u64;
+#ifdef HAVE_LINUX_EXT4_H
+#include <linux/ext4.h>
+#else
+#include <linux/types.h>
+#endif
 
 #ifndef EXT4_IOC_RESIZE_FS
 #define EXT4_IOC_RESIZE_FS		_IOW('f', 16, __u64)
-- 
2.39.1