pf: Fix memory leak on vnet shutdown or unload

Rules are unlinked in shutdown_pf(), so we must call
pf_unload_vnet_purge(), which frees unlinked rules, after that, not
before.

Reviewed by:	eri, bz
Differential Revision:	https://reviews.freebsd.org/D10040

sys/netpfil/pf/pf_ioctl.c

@@ -3766,12 +3766,12 @@ pf_unload_vnet(void)
 		return;
 	}
 
-	pf_unload_vnet_purge();
-
 	PF_RULES_WLOCK();
 	shutdown_pf();
 	PF_RULES_WUNLOCK();
 
+	pf_unload_vnet_purge();
+
 	pf_normalize_cleanup();
 	PF_RULES_WLOCK();
 	pfi_cleanup_vnet();