inet_diag: Switch the _get_exact to work with new header

Make inet_diag_get_exact work with given header instead of calculating
one from the nl message.

The SOCK_DIAG_BY_FAMILY just passes skb's one through, the compat code
converts the old header to new one.

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