IB/hfi1: Fix missing lock/unlock in verbs drain callback

IB/hfi1: Fix missing lock/unlock in verbs drain callback

The iowait_sdma_drained() callback lacked locking to
protect the qp s_flags field.

This causes the s_flags to be out of sync
on multiple CPUs, potentially corrupting the s_flags.

Fixes: a545f5308b6c ("staging/rdma/hfi: fix CQ completion order issue")
Reviewed-by: Sebastian Sanchez <[email protected]>
Signed-off-by: Mike Marciniszyn <[email protected]>
Reviewed-by: Leon Romanovsky <[email protected]>
Signed-off-by: Doug Ledford <[email protected]>