fix: wrong GTP length when Sequence Number is enabled and QoS is disabled

[louisroyer]

Bug description

If you create a configuration that make a new GTP packet with Sequence Number but you don't provide a QER IE (which is an optional IE), the resulting GTP packet's length field is off by 4.

This makes the packet total payload shorter than indicated in the header, resulting in interoperability issues (in particular, when attempting to use a custom SMF that will push a different set of PFCP rules than Free5GC's one, and where QoS management is not yet implemented).

For exemple, when creating a gNB using go-gtp, such packet is considered malformed, and is always dropped.

Bug cause

This bug results from the following:

ext_pdu_sess_ctrl_t *ext_pdu_sess;
// ...
if (opt_flag) {
    // ...
    payload_len += ((sizeof(*gtp1opt) + sizeof(*ext_pdu_sess));
}

When ext_pdu_sess is not initialized, sizeof(*ext_pdu_sess) returns 4 and not 0.

Bug mitigation

The fix is to split the incrementation in 2 steps:

1. increment by sizeof(*gtp1opt) if an option (Sequence Number, or Extension Header) is provided;
2. ensure ext_pdu_sess is well initialized before incrementing payload_len by sizeof(*ext_pdu_sess).

[tim-ywliu]

LGTM