IB/mthca: Implement query_ah method

Implement query_ah (except for AVs which are in HCA memory).  This is
needed to implement RMPP duplicate session detection on sending side
(extraction of DGID/DLID and GRH flag from address handle).

Signed-off-by: Jack Morgenstein <jackm@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>