debugnet: Handle batches of packets from if_input

Some drivers will collect multiple mbuf chains, linked by m_nextpkt,
before passing them to upper layers. debugnet_pkt_in() didn't handle
this and would process only the first packet, typically leading to
retransmits.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 841433148101aafcec8c24ae02efb042c7dfb34b)