BUG/MAJOR: quic: uninitialized quic_conn_closed struct members

BUG/MAJOR: quic: uninitialized quic_conn_closed struct members

A quic_conn_closed struct is initialized to replace the quic_conn when the
connection enters the closing to reduce the connection memory footprint.
->max_udp_payload quic_conn_close was not initialized leading to possible
BUG_ON()s in qc_rcv_buf() when comparing the RX buf size to this payload.

->cntrs counters were alon not initialized with the only consequence
to generate wrong values for these counters.

Must be backported as far as 2.9.