sock_diag: Initial skeleton

When receiving the SOCK_DIAG_BY_FAMILY message we have to find the
handler for provided family and pass the nl message to it.

This patch describes an infrastructure to work with such nandlers
and implements stubs for AF_INET(6) ones.

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