vfs: fix unconditional write_super() call in file_fsync()

We need to check ->s_dirt before calling write_super().  It became the cause
of an unneeded write.

This bug was noticed by Sudhanshu Saxena.
Signed-off-by: NOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/sync.c

@@ -64,7 +64,7 @@ int file_fsync(struct file *filp, struct dentry *dentry, int datasync)
 	/* sync the superblock to buffers */
 	sb = inode->i_sb;
 	lock_super(sb);
-	if (sb->s_op->write_super)
+	if (sb->s_dirt && sb->s_op->write_super)
 		sb->s_op->write_super(sb);
 	unlock_super(sb);