fs/devpts/inode.c: correctly check d_alloc_name() return code in devpts_pty_new()

d_alloc_name return NULL in case error, but we expect errno in
devpts_pty_new.

Addresses http://bugzilla.openvz.org/show_bug.cgi?id=1758

Signed-off-by: Andrey Vagin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/devpts/inode.c

···
 	mutex_lock(&root->d_inode->i_mutex);
 
 	dentry = d_alloc_name(root, s);
-	if (!IS_ERR(dentry)) {
+	if (dentry) {
 		d_add(dentry, inode);
 		fsnotify_create(root->d_inode, dentry);
 	} else {