get rid of NET_LOCK in the pf purge work

get rid of NET_LOCK in the pf purge work

pf purge was moved to systqmp (to get it away from KERNEL_LOCK)
which is also used as the backend for things like intr_barrier and
sched_barrier. it is common for network cards to call intr_barrier
while holding NET_LOCK, and if pf is trying to get the NET_LOCK in
the purge tasks that are now running in systqmp, it's a deadlock.
bluhm@ hit this exact issue.

sashan@ has been working to get rid of the need for NET_LOCK in pf,
so now we can remove the NET_LOCKs here rather than create a pf
specific taskq to run these tasks in.

ok sashan@ bluhm@