BUG/MINOR: quic: Missing in flight ack eliciting packet counter decrement

BUG/MINOR: quic: Missing in flight ack eliciting packet counter decrement

The decrement was missing in quic_pktns_tx_pkts_release() called each time a
packet number space is discarded. This is not sure this bug could have an impact
during handshakes. This counter is used to cancel the timer used both for packet
detection and PTO, setting its value to null. So there could be retransmissions
or probing which could be triggered for nothing.

Must be backported to 2.6.