initramfs: "crc" cpio format and INITRAMFS_PRESERVE_MTIME

[David Disseldorp <ddiss@suse.de>]

This patchset does some minor initramfs refactoring and allows cpio
entry mtime preservation to be disabled via a new Kconfig
INITRAMFS_PRESERVE_MTIME option.
Patches 3/5 to 5/5 implement support for creation and extraction of
"crc" cpio archives, which carry file data checksums. Basic tests for
this functionality can be found at
Link: https://github.com/rapido-linux/rapido/pull/163

Changes since v4, following feedback from Matthew Wilcox:
- implement cpio "crc" archive creation and extraction
- add patch to fix gen_init_cpio short read handling
- drop now-unnecessary "crc" documentation and error msg changes

Changes since v3, following feedback from Martin Wilck:
- 4/4: keep vfs_utimes() call in do_copy() path
  + drop [PATCH v3 4/5] initramfs: use do_utime() wrapper consistently
  + add do_utime_path() helper
  + clean up timespec64 initialisation
- 4/4: move all mtime preservation logic to initramfs_mtime.h and drop
  separate .c
- 4/4: improve commit message


 init/Kconfig           | 10 +++++
 init/initramfs.c       | 89 +++++++++++++++-------------------------
 init/initramfs_mtime.h | 50 +++++++++++++++++++++++
 usr/gen_init_cpio.c    | 92 ++++++++++++++++++++++++++++++------------
 4 files changed, 159 insertions(+), 82 deletions(-)