mlx4_core: Implement SRQ object lookup from srqn

Expose a new API mlx4_srq_lookup() to retrive a SRQ based on its
number.  This API is needed in the mlx4_ib driver CQ polling logic,
when a work completion is associated with a XRC TGT QP.  Since a
target QP may redirect to more than one XRC SRQ, the srq field in the
QP has no usage and the real XRC SRQ need to be retrived using the
information from the XRCETH IB header which is placed in the HW CQE.

Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>