BUG/MEDIUM: mux-quic: do not mix qcc_io_send() return codes with pacing

BUG/MEDIUM: mux-quic: do not mix qcc_io_send() return codes with pacing

With pacing implementation, qcc_send_frames() return code has been
extended to report emission interruption due to pacing limitation. This
is used only in qcc_io_send().

However, its invokation may be skipped using 'sent_done' label. This
happens on emission failure of a STOP_SENDING or RESET_STREAM (either
memory allocation failure, or transport layer rejection). In this case,
return values are mixed as qcs_send() is wrongly compared against pacing
interruption condition. This value corresponds to the length of the last
built STREAM frames.

If by mischance the last frame was 1 byte long, qcs_send() return value
is equal to pacing interruption condition. This has several effects. If
pacing is activated, it may lead to unneeded wakeup on QUIC MUX. Worst,
if pacing is not used, a BUG_ON() crash will be triggered.

Fix this by using a different variable dedicated to qcc_send_frames()
return value. By default it is initialized to 0. This ensures that
pacing code won't be activated in case qcc_send_frames() is not used.

This must be backported up to 3.1.