MFC r292601,292654:

Since r256624 (head) we have been leaking routing table allocations
  on vnet enabled jail shutdown. Call the provided cleanup
  routines for IP versions 4 and 6 to plug these leaks.

  Sponsored by:         The FreeBSD Foundation
  Reviewed by:          gnn
  Differential Revision:https://reviews.freebsd.org/D4530

Approved by:		re (gjb)

sys/netinet/in_rmx.c

@@ -373,7 +373,7 @@ in_detachhead(void **head, int off)
 {
 
 	callout_drain(&V_rtq_timer);
-	return (1);
+	return (rn_detachhead(head));
 }
 #endif
 

sys/netinet6/in6_rmx.c

@@ -290,7 +290,7 @@ in6_detachhead(void **head, int off)
 {
 
 	callout_drain(&V_rtq_mtutimer);
-	return (1);
+	return (rn_detachhead(head));
 }
 #endif