MINOR: quic: split and rename qc_lstnr_pkt_rcv()

MINOR: quic: split and rename qc_lstnr_pkt_rcv()

This change is the following of qc_lstnr_pkt_rcv() refactoring. This
function has finally been split into several ones.

The first half is renamed quic_rx_pkt_parse(). This function is
responsible to parse a QUIC packet header and calculate the packet
length.

QUIC connection retrieval has been extracted and is now called directly
by quic_lstnr_dghdlr().

The second half of qc_lstnr_pkt_rcv() is renamed to qc_rx_pkt_handle().
This function is responsible to copy a QUIC packet content to a
quic-conn receive buffer.

A third function named qc_rx_check_closing() is responsible to detect if
the connection is already in closing state. As this requires to drop the
whole datagram, it seems justified to be in a separate function.

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

This should be backported up to 2.6.