dummynet: do not store struct ifnet pointers

The dn_pkt_tag tag contained a struct ifnet pointer. If we persist that
across NET_EPOCH boundaries (as we did in dummynet) we risk panics if
the interface is removed between the packet being enqueued and it being
dequeued.

Convert the pointer into an index/generation pair and restore it when
the packet is taken out of the queue.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D35256