freebsd_network: Fix undefined reference caused by incorrect includes.

freebsd_network: Remove bogus KASSERT.

As pointed out by halamix, root_size is set to > 0 during
radix_bitmap creation, and never changed after that. So
this is a bogus check.

Fixes #14941.

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.