mptcp: introduce MPTCP_FULL_INFO getsockopt

Some user-space applications want to monitor the subflows utilization.

Dumping the per subflow tcp_info is not enough, as the PM could close
and re-create the subflows under-the-hood, fooling the accounting.
Even checking the src/dst addresses used by each subflow could not
be enough, because new subflows could re-use the same address/port of
the just closed one.

This patch introduces a new socket option, allow dumping all the relevant
information all-at-once (everything, everywhere...), in a consistent
manner.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/388
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>