netmap: fix two panics with emulated adapter

This patch fixes 2 panics. The first one is due to the current VNET not
being set in the emulated adapter transmission path. The second one
is caused by the M_PKTHDR flag not being set when preallocated mbufs
are recycled in the transmit path.

Submitted by: aleksandr.fedorov@itglobal.com
Reviewed by: vmaffione
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20824