Handle kthread_create() failures properly in taskq_create().

The return value should be checked with IS_ERR() to deal with
possible -ENOMEM or other errors.

module/spl/spl-taskq.c

@@ -841,7 +841,7 @@ taskq_create(const char *name, int nthreads, pri_t pri,
 
 		tqt->tqt_thread = kthread_create(taskq_thread, tqt,
 		    "%s/%d", name, i);
-		if (tqt->tqt_thread) {
+		if (!IS_ERR(tqt->tqt_thread)) {
 			list_add(&tqt->tqt_thread_list, &tq->tq_thread_list);
 			set_user_nice(tqt->tqt_thread, PRIO_TO_NICE(pri));
 			wake_up_process(tqt->tqt_thread);