Avoid double reference decrement when firewalls force relooping of packets

When firewalls force a reloop of packets and the caller supplied a route the reference to the route might be reduced twice creating issues.
This is especially the scenario when a packet is looped because of operation in the firewall but the new route lookup gives a down route.

Differential Revision: https://reviews.freebsd.org/D3037
Reviewed by: gnn
Approved by: gnn(mentor)