IB/mlx5: Fix srq free in destroy qp

On destroy QP the driver walks over the relevant CQ and removes CQEs
reported for the destroyed QP.  It also frees the related SRQ entry
without checking that this is actually an SRQ-related CQE.  In case of
a CQ used for both send and receive QP, we could free SRQ entries for
send CQEs.  This patch resolves this issue by verifying that this is a
SRQ related CQE by checking the SRQ number in the CQE is not zero.

Signed-off-by: Moshe Lazer <moshel@mellanox.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>