RDMA/nes: Use flush mechanism to set status for wqe in error

When an asynchronous event occurs that requires a terminate, it is
sometimes possible to identify the wqe in error.  This change uses
flush to get this information to the poll routine.  The flush
operation puts the status into the cqe.  If this information is not
available, it continues to use the more generic flush code as before.

Signed-off-by: Don Wood <donald.e.wood@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>