allow gtp tunnel updating in libgtpnl: gtp_add_tunnel()

Hi All, In current OAI implementation, we found that there are cases SPGW is out-of-sync with MME, because of the delete_session_request is not received by the SPGW because of some errors, however MME has cleared the state of that session. Then when the UE re-attaches, a new create_session_request is sent to SPGW; and because we try to allocate the same IP for the same IMSI (local change), the SPGW will result out in trying to update an existing GTP tunnel. According to http://lxr.free-electrons.com/source/drivers/net/gtp.c#L937 and https://github.com/RoadRunnr/osmo-ggsn/blob/master/libgtnl/src/gtp-genl.c#L…, the NLM_F_EXCL flag is used to prevent updating an existing tunnel. I’m is wondering what is the reason of preventing updating? While we are actively investigating sending delete_session_request properly, as temporary workaround, I’m thinking to recovery from the out-of-sync problem, we can allow the new GTP tunnel to overwrite the old one in libgtpnl. Your suggestions are welcomed. Thanks. Regards, -Jiannan