workqueue: check the allocation of system_unbound_wq

I found a trivial bug on initialization of workqueue.
Current init_workqueues doesn't check the result of
allocation of system_unbound_wq, this should be checked
like other queues.

Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3692,7 +3692,8 @@ static int __init init_workqueues(void)
 	system_nrt_wq = alloc_workqueue("events_nrt", WQ_NON_REENTRANT, 0);
 	system_unbound_wq = alloc_workqueue("events_unbound", WQ_UNBOUND,
 					    WQ_UNBOUND_MAX_ACTIVE);
-	BUG_ON(!system_wq || !system_long_wq || !system_nrt_wq);
+	BUG_ON(!system_wq || !system_long_wq || !system_nrt_wq ||
+	       !system_unbound_wq);
 	return 0;
 }
 early_initcall(init_workqueues);