MAC/do: clone_rules(): Readability improvements, constification

Constify in order to let the compiler check that source and destination
arguments are passed in the proper order in the different calls.

No functional change (intended).

Reviewed by:    bapt
MFC after:      1 month
Sponsored by:   The FreeBSD Foundation
Pull Request:   https://ron-dev.freebsd.org/FreeBSD/src/pulls/38

sys/security/mac_do/mac_do.c

@@ -1370,27 +1370,29 @@ set_default_conf(struct prison *const pr)
 static void
 clone_rules(struct rules *const dst, const struct rules *const src)
 {
-	struct rule *src_rule, *dst_rule;
+	const struct rule *src_rule;
 
 	strlcpy(dst->string, src->string, sizeof(dst->string));
 
 	STAILQ_FOREACH(src_rule, &src->head, r_entries) {
-		dst_rule = malloc(sizeof(*dst_rule), M_MAC_DO, M_WAITOK |
-		    M_ZERO);
+		struct rule *const dst_rule = malloc(sizeof(*dst_rule),
+		    M_MAC_DO, M_WAITOK);
 		bcopy(src_rule, dst_rule, sizeof(*dst_rule));
 
 		if (src_rule->uids_nb > 0) {
-			dst_rule->uids = malloc(sizeof(*dst_rule->uids) *
-			    src_rule->uids_nb, M_MAC_DO, M_WAITOK);
-			bcopy(src_rule->uids, dst_rule->uids,
-			    sizeof(*dst_rule->uids) * src_rule->uids_nb);
+			const size_t uids_size = sizeof(*dst_rule->uids) *
+			    src_rule->uids_nb;
+
+			dst_rule->uids = malloc(uids_size, M_MAC_DO, M_WAITOK);
+			bcopy(src_rule->uids, dst_rule->uids, uids_size);
 		}
 
 		if (src_rule->gids_nb > 0) {
-			dst_rule->gids = malloc(sizeof(*dst_rule->gids) *
-			    src_rule->gids_nb, M_MAC_DO, M_WAITOK);
-			bcopy(src_rule->gids, dst_rule->gids,
-			    sizeof(*dst_rule->gids) * src_rule->gids_nb);
+			const size_t gids_size = sizeof(*dst_rule->gids) *
+			    src_rule->gids_nb;
+
+			dst_rule->gids = malloc(gids_size, M_MAC_DO, M_WAITOK);
+			bcopy(src_rule->gids, dst_rule->gids, gids_size);
 		}
 
 		STAILQ_INSERT_TAIL(&dst->head, dst_rule, r_entries);