netmap: netmap_transmit should honor bpf packet tap hook

This allows tcpdump to capture outbound kernel packets while
in netmap mode

Submitted by: Marc de la Gueronniere <mdelagueronniere@verisign.com>
Reviewed by: vmaffione
MFC after: 1 week
Sponsored by: Verisign, Inc.
Differential Revision: https://reviews.freebsd.org/D17896