ovl: ovl_obtain_alias(): don't call d_instantiate_anon() for old

The situation is the same as for __d_obtain_alias() (which is what that
thing is parallel to) - if we find a preexisting alias, we want to grab it,
drop the inode and return the alias we'd found.

The only thing d_instantiate_anon() does compared to that is spurious
security_d_instiate() that has already been done to that dentry with exact
same arguments.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/export.c

@@ -308,9 +308,11 @@ static struct dentry *ovl_obtain_alias(struct super_block *sb,
 		ovl_set_flag(OVL_UPPERDATA, inode);
 
 	dentry = d_find_any_alias(inode);
-	if (!dentry) {
+	if (dentry)
+		goto out_iput;
+
 	dentry = d_alloc_anon(inode->i_sb);
-		if (!dentry)
+	if (unlikely(!dentry))
 		goto nomem;
 	oe = ovl_alloc_entry(lower ? 1 : 0);
 	if (!oe)
@@ -323,16 +325,18 @@ static struct dentry *ovl_obtain_alias(struct super_block *sb,
 	dentry->d_fsdata = oe;
 	if (upper_alias)
 		ovl_dentry_set_upper_alias(dentry);
-	}
+
 	ovl_dentry_update_reval(dentry, upper,
 			DCACHE_OP_REVALIDATE | DCACHE_OP_WEAK_REVALIDATE);
 
 	return d_instantiate_anon(dentry, inode);
 
 nomem:
-	iput(inode);
 	dput(dentry);
-	return ERR_PTR(-ENOMEM);
+	dentry = ERR_PTR(-ENOMEM);
+out_iput:
+	iput(inode);
+	return dentry;
 }
 
 /* Get the upper or lower dentry in stach whose on layer @idx */