nfsd: fix an IS_ERR() vs NULL check

The get_backchannel_cred() used to return error pointers on error but
now it returns NULL pointers.

Fixes: 97f68c6b ("SUNRPC: add 'struct cred *' to auth_cred and rpc_cre")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4callback.c

@@ -913,9 +913,9 @@ static int setup_callback_client(struct nfs4_client *clp, struct nfs4_cb_conn *c
 		return PTR_ERR(client);
 	}
 	cred = get_backchannel_cred(clp, client, ses);
-	if (IS_ERR(cred)) {
+	if (!cred) {
 		rpc_shutdown_client(client);
-		return PTR_ERR(cred);
+		return -ENOMEM;
 	}
 	clp->cl_cb_client = client;
 	clp->cl_cb_cred = cred;