dummynet: use m_rcvif_serialize/restore when queueing packets

This fixed panic with interface being removed while packet
was sitting on a queue. This allows to pass all dummynet
tests including forthcoming dummynet:ipfw_interface_removal
and dummynet:pf_interface_removal and demonstrates use of
m_rcvif_serialize() and m_rcvif_restore().

Reviewed by: kp
Differential revision: https://reviews.freebsd.org/D33267