inet_diag: Introduce socket family checks

The new API will specify family to work with. Teach the existing
socket walking code to bypass not interesting ones.

To preserve compatibility with existing behavior the _compat code
sets interesting family to AF_UNSPEC to dump them all.

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