Conditionalize an in_epoch() call on INVARIANTS.

Its result is only used to determine whether to perform further
INVARIANTS-only checks.  Remove a stale comment while here.

Submitted by:	Sebastian Huber <sebastian.huber@embedded-brains.de>
MFC after:	1 week

sys/net/if_lagg.c

@@ -1955,12 +1955,10 @@ lagg_link_active(struct lagg_softc *sc, struct lagg_port *lp)
 	 * Search a port which reports an active link state.
 	 */
 
-	/*
-	 * This is called with either LAGG_RLOCK() held or
-	 * LAGG_XLOCK(sc) held.
-	 */
+#ifdef INVARIANTS
 	if (!in_epoch(net_epoch_preempt))
 		LAGG_XLOCK_ASSERT(sc);
+#endif
 
 	if (lp == NULL)
 		goto search;