BUG/MINOR: quic: ensure a detached coalesced packet can't access its neighbours

BUG/MINOR: quic: ensure a detached coalesced packet can't access its neighbours

Reset ->prev and ->next fields of a coalesced TX packet to ensure it cannot access
several times its neighbours after it is supposed to be detached from them calling
quic_tx_packet_dgram_detach().

There are two cases where a packet can be coalesced to another previous built one:
this is when it is built into the same datagrame without GSO (and flagged flag with
QUIC_FL_TX_PACKET_COALESCED) or when sent from the same sendto() syscall with GOS
(not flagged with QUIC_FL_TX_PACKET_COALESCED).

This fix may be in relation with GH #2839.

Must be backported as far as 2.6.