revert pf.c r1.1152 again: move pf_purge out from under the kernel lock

revert pf.c r1.1152 again: move pf_purge out from under the kernel lock

Using systqmp for pf_purge creates a deadlock between pf_purge()
and ixgbe_stop() and possibly other drivers.  On systqmp pf(4) needs
netlock which the interface ioctl(2) is holding.  ix(4) waits in
sched_barrier() which is also scheduled on the systqmp task queue.

Removing the netlock from pf_purge() as a quick fix caused other
problems.

backout suggested by deraadt@