packet: Report rings cfg via diag engine

One extension bit may result in two nlattrs -- one per ring type.
If some ring type is not configured, then the respective nlatts
will be empty.

The structure reported contains the data, that is given to the
corresponding ring setup socket option.

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