Add ip6_tryforward() - a run to completion forwarding implementation
for IPv6.

It gets performance benefits from reduced number of checks. It doesn't
copy mbuf to be able send ICMPv6 error message, because it keeps mbuf
unchanged until the moment, when the route decision has been made.
It doesn't do IPsec checks, and when some IPsec security policies present,
ip6_input() uses normal slow path.

Reviewed by: bz, gnn
Obtained from: Yandex LLC
MFC after: 1 month
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D8527