routing: fix panic triggered by the 'gr_idx != 0' assert in nhg code

Nexthop groups can be referenced by the external code. The reference
 can be released after the VNET destruction. Furthermore, nexthop
 groups use a single per-rib lock, which is destroyed during the
 VNET desctruction. To eliminate use-after-free problem, each nhg
 is marked as "unlinked" during the VNET destruction stage, leaving
 nhg_idx intact. Normally there should not be such nexthops, but if
 there are any, the kernel will panic on 'gr_idx != 0' when the
 last nhg reference is released.

Address this by using the assert checks only when the nexthop group
 is destroyed during "valid" VNET lifetime.

MFC after:	3 days

sys/net/route/nhgrp_ctl.c

@@ -382,10 +382,11 @@ nhgrp_free(struct nhgrp_object *nhg)
 			NET_EPOCH_EXIT(et);
 			return;
 		}
+		MPASS((nhg_priv->nhg_idx == 0));
+		MPASS((nhg_priv->nhg_refcount == 0));
 	}
 	NET_EPOCH_EXIT(et);
 
-	KASSERT((nhg_priv->nhg_idx == 0), ("gr_idx != 0"));
 	NET_EPOCH_CALL(destroy_nhgrp_epoch, &nhg_priv->nhg_epoch_ctx);
 }
 
@@ -402,10 +403,6 @@ destroy_nhgrp_int(struct nhgrp_priv *nhg_priv)
 __noinline static void
 destroy_nhgrp(struct nhgrp_priv *nhg_priv)
 {
-
-	KASSERT((nhg_priv->nhg_refcount == 0), ("nhg_refcount != 0"));
-	KASSERT((nhg_priv->nhg_idx == 0), ("gr_idx != 0"));
-
 	IF_DEBUG_LEVEL(LOG_DEBUG2) {
 		char nhgbuf[NHOP_PRINT_BUFSIZE] __unused;
 		FIB_NH_LOG(LOG_DEBUG2, nhg_priv->nhg_nh_weights[0].nh,