dlm: check if workqueues are NULL before flushing/destroying

If the DLM lowcomms stack is shut down before any DLM
traffic can be generated, flush_workqueue() and
destroy_workqueue() can be called on empty send and/or recv
workqueues.

Insert guard conditionals to only call flush_workqueue()
and destroy_workqueue() on workqueues that are not NULL.
Signed-off-by: David Windsor <dwindsor@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>

fs/dlm/lowcomms.c

@@ -1628,7 +1628,9 @@ static void clean_writequeues(void)
 
 static void work_stop(void)
 {
+	if (recv_workqueue)
 		destroy_workqueue(recv_workqueue);
+	if (send_workqueue)
 		destroy_workqueue(send_workqueue);
 }
 
@@ -1689,12 +1691,16 @@ static void work_flush(void)
 	struct hlist_node *n;
 	struct connection *con;
 
+	if (recv_workqueue)
 		flush_workqueue(recv_workqueue);
+	if (send_workqueue)
 		flush_workqueue(send_workqueue);
 	do {
 		ok = 1;
 		foreach_conn(stop_conn);
+		if (recv_workqueue)
 			flush_workqueue(recv_workqueue);
+		if (send_workqueue)
 			flush_workqueue(send_workqueue);
 		for (i = 0; i < CONN_HASH_SIZE && ok; i++) {
 			hlist_for_each_entry_safe(con, n,