Turn `pflow_softc' list into SMR list.

Turn `pflow_softc' list into SMR list.

Since the revision 1.1182 of net/pf.c netlock is not taken while
export_pflow() called from pf_purge_states(). Current locks order
requires netlock to be taken before PF_LOCK(), so there is no reason
to turn it back into this path only for optional export_pflow() call.

The `pflowif_list' foreach loop has no context switch within, so SMR
list is better than mutex(9).

Tested by Hrvoje Popovski.

ok sashan bluhm