Fix a race condition where a mutex was destroyed while sleeping on it.

Found while analyzing a report from julian. It might fix his bug.
Approved by: rrs (mentor)
MFC after: 3 days

sys/netinet/sctp_bsd_addr.c

@@ -97,6 +97,7 @@ sctp_iterator_thread(void *v)
 		    &SCTP_BASE_INFO(ipi_iterator_wq_mtx),
 		    0, "waiting_for_work", 0);
 		if (SCTP_BASE_INFO(threads_must_exit)) {
+			SCTP_IPI_ITERATOR_WQ_DESTROY();
 			kthread_exit();
 		}
 		sctp_iterator_worker();

sys/netinet/sctp_pcb.c

@@ -5616,7 +5616,6 @@ sctp_pcb_finish(void)
 	SCTP_IP_PKTLOG_DESTROY();
 
 #endif
-	SCTP_IPI_ITERATOR_WQ_DESTROY();
 	SCTP_IPI_ADDR_DESTROY();
 	SCTP_ITERATOR_LOCK_DESTROY();
 	SCTP_STATLOG_DESTROY();