Generalize the inet_diag infrastructure

This is an attempt to prepare the existing inet_diag infrastructure to work with arbitrary address family and protocol pair, not only AF_INET(6) and TCP/DCCP.

This set adds the sock_diag infrastructure, that handles the new SOCK_DIAG_BY_FAMILY message type by the NETLINK_INET_DIAG socket. The core API struct for this (called sock_diag_req) contains two fields - family and type - which specify which sockets to dump. Additional information is considered to be family handler specific.

The existing inet_diag code is patched to register himself inside the sock_diag and provide callbacks for dumping AF_INET/AF_INET6 IPPROTO_TCP/IPPROTO_DCCP sockets. The existing API facilities such as state filtering and bytecode is of course preserved and is treated by inet_diag specific data.

Binary compatibility with existing NETLINK_INET_DIAG is also kept.

No additional stuff in this set. The support for IPPROTO_UDP protocol and AF_UNIX family will come later if this set is approved.

Applies to net-next, patch for iproute2's ss tool (proving that the new code ... pretends to work) is at the end of the thread.