fat: make fat_update_time get its own timestamp

In later patches, we're going to drop the "now" parameter from the
update_time operation. Fix fat_update_time to fetch its own timestamp.
It turns out that this is easily done by just passing a NULL timestamp
pointer to fat_truncate_time.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Frank Sorenson <sorenson@redhat.com>
Message-Id: <20230810-ctime-fat-v1-2-327598fd1de8@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>