domainpolicy: use set_force_socket()

- use set_force_socket() instead of msg->force_send_socket
(cherry picked from commit 67b7508a1c8964abbf55f3f09363a45f8aa871f8)

modules_k/domainpolicy/domainpolicy.c

@@ -850,7 +850,7 @@ int dp_apply_policy(struct sip_msg* _msg, char* _s1, char* _s2) {
 		}
 		si = grep_sock_info( &host, (unsigned short) port, (unsigned short) proto);
 		if (si) {
-			_msg->force_send_socket = si;
+			set_force_socket(_msg, si);
 		} else {
 			LM_WARN("could not find socket for"
 					"send_socket '%.*s'\n", val.s.len, ZSW(val.s.s));