MINOR: quic: extract connection retrieval

MINOR: quic: extract connection retrieval

Simplify qc_lstnr_pkt_rcv() by extracting code responsible to retrieve
the quic-conn instance. This code is put in a dedicated function named
quic_rx_pkt_retrieve_conn(). This new function could be skipped if a
FD-owned quic-conn socket is used.

The first traces of qc_lstnr_pkt_rcv() have been clean up as qc instance
is always NULL here : thus qc parameter can be removed without any
change.

This change has no functional impact. It is a part of a refactoring
series on qc_lstnr_pkt_rcv(). The objective is facilitate integration of
FD-owned socket patches.

This should be backported up to 2.6.