Register pfil hooks when VNET != vnet0. r302298, which virtualized ipf,
assumed the pfil hook registration performed in ipf_modload() would take
care of this. However ipf_modload() is only called when the ipl kld is
loaded or when ipfilter is first called when it is statically linked
into the kernel at build time.

Prior to this, even though r302298 has been in the tree for a while, it
has never been used. So, r302298 in reality begins now.

PR: 212000
Reported by: ahsanb@
MFC after: 1 month