ip_mroute: Use NET_EPOCH_WAIT() macro

This makes it easier to grep the usage.

Reviewed by:	kp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D45715

(cherry picked from commit 08a98731dd)

sys/netinet/ip_mroute.c

@@ -766,7 +766,7 @@ X_ip_mrouter_done(void)
 	 * Wait for all epoch sections to complete to ensure
 	 * V_ip_mrouter = NULL is visible to others.
 	 */
-	epoch_wait_preempt(net_epoch_preempt);
+	NET_EPOCH_WAIT();
 
 	/* Stop and drain task queue */
 	taskqueue_block(V_task_queue);