xfs: have xfs_vn_update_time gets its own timestamp

In later patches we're going to drop the "now" parameter from the
update_time operation. Prepare XFS for this by reworking how it fetches
timestamps and sets them in the inode. Ensure that we update the ctime
even if only S_MTIME is set.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Acked-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230807-mgctime-v7-7-d1dec143a704@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>