from the inet6 stack along with statistics and make sure we
properly free the rt in all cases.
While the current situation is not better performance wise it
prevents panics seen more often these days.
After more inet6 and ipsec cleanup we should be able to improve
the situation again passing the rt to ip6_forward directly.
Leave the ip6_forward_rt entry in struct vinet6 but mark it
for removal.
PR: kern/128247, kern/131038
MFC after: 25 days
Committed from: Bugathon #6
Tested by: Denis Ahrens <denis@h3q.com> (different initial version)
09f8c3ff36