mptcp: listen diag dump support

mptcp: listen diag dump support

makes 'ss -Ml' show mptcp listen sockets.

Iterate over the tcp listen sockets and pick those that have mptcp ulp
info attached.

mptcp_diag_get_info() is modified to prefer msk->first for mptcp sockets
in listen state.  This reports accurate number for recv and send queue
(pending / max connection backlog counters).

Sample output:
ss -Mil
State        Recv-Q Send-Q Local Address:Port  Peer Address:Port
LISTEN       0      20     127.0.0.1:12000     0.0.0.0:*
         subflows_max:2

Signed-off-by: Florian Westphal <[email protected]>
Signed-off-by: Mat Martineau <[email protected]>
Signed-off-by: David S. Miller <[email protected]>