l2tp: Support several sockets with same IP/port quadruple

Some l2tp providers will use 1701 as origin port and open several
tunnels for the same origin and target. On the Linux side, this
may mean opening several sockets, but then trafic will go to only
one of them, losing the trafic for the tunnel of the other socket
(or leaving it up to userland, consuming a lot of cpu%).

This can also happen when the l2tp provider uses a cluster, and
load-balancing happens to migrate from one origin IP to another one,
for which a socket was already established. Managing reassigning
tunnels from one socket to another would be very hairy for userland.

Lastly, as documented in l2tpconfig(1), as client it may be necessary
to use 1701 as origin port for odd firewalls reasons, which could
prevent from establishing several tunnels to a l2tp server, for the
same reason: trafic would get only on one of the two sockets.

With the V2 protocol it is however easy to route trafic to the proper
tunnel, by looking up the tunnel number in the network namespace. This
fixes the three cases altogether.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20240506215336.1470009-1-samuel.thibault@ens-lyon.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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