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>

fs/fat/misc.c

@@ -347,7 +347,7 @@ int fat_update_time(struct inode *inode, struct timespec64 *now, int flags)
 		return 0;
 
 	if (flags & (S_ATIME | S_CTIME | S_MTIME)) {
-		fat_truncate_time(inode, now, flags);
+		fat_truncate_time(inode, NULL, flags);
 		if (inode->i_sb->s_flags & SB_LAZYTIME)
 			dirty_flags |= I_DIRTY_TIME;
 		else