Resolve ipfilter kld unload issues related to VNET jails.

Resolve ipfilter kld unload issues related to VNET jails.

When the ipfilter kld is loaded, used within VNET jail, and unloaded,
then subsequent loading, use, and unloading of another packet filters
will cause the subsequently loaded netpfil kld's to panic.

The scenario is as follows:

cd /usr/tests/sys/netpfil/common

kldunload ipl
kldunload pfsync
kldunload ipfw

kyua test pass_block

kldload ipl
kyua test pass_block
kldunload ipl

kldload pfsync
kyua test pass_block
kldunload pfsync
-- page fault panic occurs here --

Reported by:	"Ahsan Barkati" <ahsanbarkati@g.....com> via kp@
Discussed with:	kp@
Tested by:	kp@
MFC after:	3 days