L2TP: Fix UDP conversation handling.

RFCs 2661 and 3931 say that L2TPv2 and L2TPv3 use a TFTP-like method
of selecting ports. The initiator picks a source port (which may or
may not be 1701, the IANA assigned L2TP port), and sends a message to
1701; the recipient picks a free port (which may or may not be 1701)
and replies to the initiator's chosen port and address, and the
conversation from then on uses the chosen ports.

In practice, due to NAT, firewalls, etc., most implementations just
use a symmetric predetermined L2TP port. To support both methods
we use one-sided conversations with one port omitted. Fix the lookup
of the reverse conversation. Part of #16565.

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