ifnet/mbuf: provide KPI to serialize/restore m->m_pkthdr.rcvif

Supplement ifindex table with generation count and use it to
serialize & restore an ifnet pointer.

Reviewed by: kp
Differential revision: https://reviews.freebsd.org/D33266
Fun note: git show e6abef09187a

(cherry picked from commit e1882428dcbbafd2814d7e17b977a8f686784b39)