erofs: rename ctime to mtime

EROFS images should inherit modification time rather than change time,
since users and host tooling have no easy way to control change time.

To reflect the new timestamp meaning, i_ctime and i_ctime_nsec are
renamed to i_mtime and i_mtime_nsec.

Link: https://lore.kernel.org/r/20220311041829.3109511-1-dvander@google.com

 # v1
Signed-off-by: David Anderson <dvander@google.com>
[ Gao Xiang: update document as well. ]
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220317114959.106787-1-hsiangkao@linux.alibaba.com

 # v2
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>