inet_diag: Introduce new inet_diag_req header

This one coinsides with the sock_diag_req in the beginning and
contains only used fields from its previous analogue.

The existing code is patched to use the _compat version of it
for now.

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