BUG/MINOR: quic: Avoid BUG_ON() on ->on_pkt_lost() BBR callback call

BUG/MINOR: quic: Avoid BUG_ON() on ->on_pkt_lost() BBR callback call

The per-packet delivery rate sample is applied to ack-eliciting packet only
calling ->drs_on_transmit() BBR callback. So, ->on_pkt_lost() which inspects the
delivery rate sampling information during packet loss detection must not be
called for non ack-eliciting packet. If not, it would be facing with non
initialized variables with big chance to trigger a BUG_ON().

As BBR is implemented in the current developement version, there is
no need to backport this patch.