Make deletion of RoCE GID entries synchronous in ibcore.

When a network device is departing, the RoCE GID entries should be
cleared before the default L2 link layer address is freed. Else a NULL
pointer access may happen.

MFC after:	1 week
Sponsored by:	Mellanox Technologies

sys/ofed/drivers/infiniband/core/ib_roce_gid_mgmt.c

@@ -362,6 +362,9 @@ roce_gid_delete_all_event(struct net_device *ndev)
 	dev_hold(ndev);
 	work->ndev = ndev;
 	queue_work(roce_gid_mgmt_wq, &work->work);
+
+	/* make sure job is complete before returning */
+	flush_workqueue(roce_gid_mgmt_wq);
 }
 
 static int