MINOR: quic: extend pn_offset field from quic_rx_packet

MINOR: quic: extend pn_offset field from quic_rx_packet

pn_offset field was only set if header protection cannot be removed.
Extend the usage of this field : it is now set everytime on packet
parsing in qc_lstnr_pkt_rcv().

This change helps to clean up API of Rx functions by removing
unnecessary variables and function argument.

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.