fat: ignore ctime updates, and keep ctime identical to mtime in memory

FAT supports creation time but not change time, and there was no
corresponding timestamp for creation time in previous VFS.  The original
implementation took the compromise of saving the in-memory change time
into the on-disk creation time field, but this would lead to compatibility
issues with non-linux systems.

To address this issue, this patch changes the behavior of ctime.  It will
no longer be loaded and stored from the creation time on disk.  Instead of
that, it'll be consistent with the in-memory mtime and share the same
on-disk field.  All updates to mtime will also be applied to ctime in
memory, while all updates to ctime will be ignored.

Link: https://lkml.kernel.org/r/20220503152536.2503003-2-cccheng@synology.com


Signed-off-by: Chung-Chiang Cheng <cccheng@synology.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>