Reintroduce the IP rule&route for OAI UE through setup_ue_ipv4_route()

These rules & route are necessary to properly send packets on oaitun_ue1
interface:

1. This forces the packets coming from a subnet different than the UE's
   subnet to go back through oaitun_ue1 rather than via what the default
   route defined on the system (e.g., important if ping from the
   internet arrives).

2. On machines setting net.ipv4.conf.oaitun_ue1.rp_filter=1 (e.g.
   RHEL), this prevents that source filtering for packets coming back is
   applied and those packets be dropped. By default, many hosts have
   rp_filter=2, so no strict source filtering is applied, and it would
   work; on others, this rule prevents source filter dropping.

openair3/NAS/COMMON/ESM/MSG/PduSessionEstablishmentAccept.c

@@ -119,6 +119,7 @@ void capture_pdu_session_establishment_accept_msg(uint8_t *buffer, uint32_t msg_
                    addr->pdu_addr_oct3,
                    addr->pdu_addr_oct4);
           nas_config(1, ip, "oaitun_ue");
+          setup_ue_ipv4_route(1, ip, "oaitun_ue");
           LOG_T(NAS, "PDU SESSION ESTABLISHMENT ACCEPT - Received UE IP: %s\n", ip);
         } else {
           curPtr += psea_msg.pdu_addr_ie.pdu_length;