freebsd_network: Fix and implement some mbuf functions.

Spotted while reviewing the code during iflib porting.

freebsd11_network -> freebsd_network.

FreeBSD 12 has no major changes to the ifnet KPIs that constitute a
source compatibility break, save a single one related to locking
which doesn't really apply to us, and so we don't need to create
a "freebsd12_network" directory to work through the upgrades.