pf: Fix vnet initialisation

When running the vnet init code (pf_load_vnet()) we used to iterate over
all vnets, marking them as unhooked.
This is incorrect and leads to panics if pf is unloaded, as the unload
code does not unregister the pfil hooks (because the vnet is marked as
unhooked).

There's no need or reason to touch other vnets during initialisation.
Their pf_load_vnet() function will be triggered, which handles all
required initialisation.

Reviewed by: zec, gnn
Differential Revision: https://reviews.freebsd.org/D10592