9p: Correct fidpool creation failure in p9_client_create

On error, p9_idpool_create returns an ERR_PTR-encoded errno.

Signed-off-by: Josef 'Jeff' Sipek <[e-mail]>
Acked-by: Eric Van Hensbergen <[e-mail]>

net/9p/client.c

···
 	spin_lock_init(&clnt->lock);
 	INIT_LIST_HEAD(&clnt->fidlist);
 	clnt->fidpool = p9_idpool_create();
-	if (!clnt->fidpool) {
+	if (IS_ERR(clnt->fidpool)) {
 		err = PTR_ERR(clnt->fidpool);
 		clnt->fidpool = NULL;
 		goto error;