pf: Do not lose the VNET lock when ending the purge thread

When the pf_purge_thread() exits it must make sure to release the
VNET_LIST_RLOCK it still holds.
kproc_exit() does not return.

sys/netpfil/pf/pf.c

@@ -1440,6 +1440,7 @@ pf_purge_thread(void *unused __unused)
 			if (pf_end_threads) {
 				pf_end_threads++;
 				wakeup(pf_purge_thread);
+				VNET_LIST_RUNLOCK();
 				kproc_exit(0);
 			}