CLEANUP: quic: no need for atomics on packet refcnt

CLEANUP: quic: no need for atomics on packet refcnt

This is a leftover from the implementation's history, but the
quic_rx_packet and quic_tx_packet ref counts were still atomically
updated. It was found in perf top that the cost of the atomic inc
in quic_tx_packet_refinc() alone was responsible for 1% of the CPU
usage at 135 Gbps. Given that packets are only processed on their
assigned thread we don't need that anymore and this can be replaced
with regular non-atomic operations.

Doing this alone has reduced the CPU usage of qc_do_build_pkt()
from 3.6 to 2.5% and increased the overall bit rate by about 1%.