BUG/MINOR: quic: fix padding of INITIAL packets

BUG/MINOR: quic: fix padding of INITIAL packets

API for sending has been extended to support emission on more than 2 QEL
instances. However, this has rendered the PADDING emission for INITIAL
packets less previsible. Indeed, if qc_send() is used with empty QEL
instances, a padding frame may be generated before handling the last QEL
registered, which could cause unnecessary padding to be emitted.

This commit simplify PADDING by only activating it for the last QEL
registered. This ensures that no superfluous padding is generated as if
the minimal INITIAL datagram length is reached, padding is resetted
before handling last QEL instance.

This bug is labelled as minor as haproxy already emit big enough INITIAL
packets coalesced with HANDSHAKE one without needing padding. This
however render the padding code difficult to test. Thus, it may be
useful to force emission on INITIAL qel only without coalescing
HANDSHAKE packet. Here is a sample to reproduce it :

--- a/src/quic_conn.c
+++ b/src/quic_conn.c
@@ -794,6 +794,14 @@ struct task *quic_conn_io_cb(struct task *t, void *context, unsigned int state)
                }
        }

+       if (qc->iel && qel_need_sending(qc->iel, qc)) {
+               struct list empty = LIST_HEAD_INIT(empty);
+               qel_register_send(&send_list, qc->iel, &qc->iel->pktns->tx.frms);
+               if (qc->hel)
+                       qel_register_send(&send_list, qc->hel, &empty);
+               qc_send(qc, 0, &send_list);
+       }
+
        /* Insert each QEL into sending list if needed. */
        list_for_each_entry(qel, &qc->qel_list, list) {
                if (qel_need_sending(qel, qc))

This should be backported up to 3.0.

(cherry picked from commit a60609f1aa3e5f61d2a2286fdb40ebf6936a80ee)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>