Changes

tunmap: always set GTP-U source port to 2152 when forwarding

We see GTP-U originating from ports other than 2152 in the field. When
osmo-upf forwards these, we want to forward from our GTP-U port 2152,
since that is the only port osmo-upf has bound for GTP-U (for echo).

According to 3GPP TS 29.060, the *destination* port for GTP-U shall be
2152 -- but the source port is apparently allowed to be different.

Before this patch, we would forward GTP-U like this:

  3.3.3.3:33333 -> (3.3.3.4:2152  UPF  2.2.2.2:33333) -> 1.1.1.1:2152
                                               ^^^^^

Instead we want to always send from UDP source port 2152:

  3.3.3.3:33333 -> (3.3.3.4:2152  UPF  2.2.2.2:2152) -> 1.1.1.1:2152
                                               ^^^^

This hasn't shown up before because so far all GTP-U peers we saw
consistently used source port 2152.

Related: SYS#6773
Change-Id: Idaf43f1c2b915846b50a8b97305f0229e34ad539