fat: Kill d_invalidate() in vfat_lookup()

d_invalidate() for positive dentry doesn't work in some cases
(vfsmount, nfsd, and maybe others). shrink_dcache_parent() by
d_invalidate() is pointless for vfat usage at all.

So, this kills it, and intead of it uses d_move().

To save old behavior, this returns alias simply for directory (don't
change pwd, etc..). the directory lookup shouldn't be important for
performance.
Signed-off-by: NOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/fat/namei_vfat.c

@@ -745,14 +745,13 @@ static struct dentry *vfat_lookup(struct inode *dir, struct dentry *dentry,
 		 *
 		 * Switch to new one for reason of locality if possible.
 		 */
-		if (d_invalidate(alias) == 0)
-			dput(alias);
-		else {
+		BUG_ON(d_unhashed(alias));
+		if (!S_ISDIR(inode->i_mode))
+			d_move(alias, dentry);
 		iput(inode);
 		unlock_super(sb);
 		return alias;
 	}
-	}
 out:
 	unlock_super(sb);
 	dentry->d_op = sb->s_root->d_op;