mbuf(9): Implement a leaf network interface field in the mbuf packet header.

When packets are received they may traverse several network interfaces like
vlan(4) and lagg(9). When doing receive side offloads it is important to
know the first network interface entry point, because that is where all
offloading is taking place. This makes it possible to track receive
side route changes for multiport setups, for example when lagg(9) receives
traffic from more than one port. This avoids having to install multiple
offloading rules for the same stream.

This field works similar to the existing "rcvif" mbuf packet header field.

Submitted by: jhb@
Reviewed by: gallatin@ and gnn@
Differential revision: https://reviews.freebsd.org/D35339
Sponsored by: NVIDIA Networking
Sponsored by: Netflix