pf syncookies: fix memory leak

We forgot to free the nvlist (and packed nvlist) on success.
While here start using the ERROUT macro to clean up error handling, and
to add SDTs for better debugging.

Reported by:	Coverity
CID:		1473150

sys/netpfil/pf/pf_syncookies.c

@@ -141,10 +141,13 @@ pf_get_syncookies(struct pfioc_nv *nv)
 {
 	nvlist_t	*nvl = NULL;
 	void		*nvlpacked = NULL;
+	int		 error;
+
+#define ERROUT(x)	ERROUT_FUNCTION(errout, x)
 
 	nvl = nvlist_create(0);
 	if (nvl == NULL)
-		return (ENOMEM);
+		ERROUT(ENOMEM);
 
 	nvlist_add_bool(nvl, "enabled",
 	    V_pf_status.syncookies_mode != PF_SYNCOOKIES_NEVER);
@@ -154,21 +157,23 @@ pf_get_syncookies(struct pfioc_nv *nv)
 	nvlist_add_number(nvl, "lowwater", V_pf_syncookie_status.lowat);
 
 	nvlpacked = nvlist_pack(nvl, &nv->len);
-	if (nvlpacked == NULL) {
-		nvlist_destroy(nvl);
-		return (ENOMEM);
-	}
+	if (nvlpacked == NULL)
+		ERROUT(ENOMEM);
+
 	if (nv->size == 0) {
-		nvlist_destroy(nvl);
-		free(nvlpacked, M_TEMP);
-		return (0);
+		ERROUT(0);
 	} else if (nv->size < nv->len) {
-		nvlist_destroy(nvl);
-		free(nvlpacked, M_TEMP);
-		return (ENOSPC);
+		ERROUT(ENOSPC);
 	}
 
-	return (copyout(nvlpacked, nv->data, nv->len));
+	error = copyout(nvlpacked, nv->data, nv->len);
+
+#undef ERROUT
+errout:
+	nvlist_destroy(nvl);
+	free(nvlpacked, M_TEMP);
+
+	return (error);
 }
 
 int