Modify IPv6 statistic accounting in ip6_input().

Add rcvif local variable to keep inbound interface pointer. Count
ifs6_in_discard errors in all "goto bad" cases. Now it will count
errors even if mbuf was freed. Modify all places where m->m_pkthdr.rcvif
is used to use local rcvif variable.

Obtained from: Yandex LLC
MFC after: 1 month