BUG/MAJOR: quic: fix INITIAL padding with probing packet only

BUG/MAJOR: quic: fix INITIAL padding with probing packet only

A QUIC datagram that contains an INITIAL packet must be padded to 1.200
bytes to prevent any deadlock due to anti-amplification protection. This
is implemented by encoding a PADDING frame on the last packet of the
datagram if necessary.

Previously, qc_prep_pkts() was responsible to activate padding when
calling qc_do_build_pkt(), as it knows which packet is the last to
encode. However, this has the side-effect of preventing PING emission
for probing with no data as this case was handled in an else-if branch
after padding. This was fixed by the below commit

  217e467e89d15f3c22e11fe144458afbf718c8a8
  BUG/MINOR: quic: fix malformed probing packet building

Above logic was altered to fix the PING case : padding was set to false
explicitely in qc_prep_pkts(). Padding was then added in a specific
block dedicated to the PING case in qc_do_build_pkt() itself for INITIAL
packets.

However, the fix is incorrect if the last QEL used to built a packet is
not the initial one and probing is used with PING frame only. In this
case, specific block in qc_do_build_pkt() does not add padding. This
causes a BUG_ON() crash in qc_txb_store() which catches these packets as
irregularly formed.

To fix this while also properly handling PING emission, revert to the
original padding logic : qc_prep_pkts() is responsible to activate
INITIAL padding. To not interfere with PING emission, qc_do_build_pkt()
body is adjusted so that PING block is moved up in the function and
detached from the padding condition.

The main benefit from this patch is that INITIAL padding decision in
qc_prep_pkts() is clearer now.

Note that padding can also be activated by qc_do_build_pkt(), as packets
should be big enough for header protection decipher. However, this case
is different from INITIAL padding, so it is not covered by this patch.

This should be backported up to 2.6.