netmap: Handle packet batches in generic mode

ifnets are allowed to pass batches of multiple packets to if_input,
linked by the m_nextpkt pointer. iflib_rxeof() sometimes does this, for
example. Netmap's generic mode did not handle this and would only
deliver the first packet in the batch, leaking the rest.

PR: 270636
Reviewed by: vmaffione
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39426