packet: Report socket mclist info via diag module

The info is reported as an array of packet_diag_mclist structures. Each
includes not only the directly configured values (index, type, etc), but
also the "count".

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>