scsi: vhost-scsi: Report direction completion support

This has vhost-scsi report that it supports direct completions.

When using a worker task per queue or group of queues with fast backends
then enabling direct completion and submissions increases performance
20-30% with workloads like:

  fio --filename=/dev/sdb  --direct=1 --rw=randrw --bs=8K \
      --ioengine=libaio --iodepth=128  --numjobs=$jobs

As jobs matches and passes the number of vCPUs in the VM then the
benefit increases.

However, when using a single worker then queueing completions and
submissions is best as the worker is busy handling mapping data and
setting/tearing down commands.

Signed-off-by: Mike Christie <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Martin K. Petersen <[email protected]>

drivers/vhost/scsi.c

@@ -2951,6 +2951,7 @@ static const struct target_core_fabric_ops vhost_scsi_ops = {
 	.tfc_tpg_attrib_attrs		= vhost_scsi_tpg_attrib_attrs,
 
 	.default_compl_type		= TARGET_QUEUE_COMPL,
+	.direct_compl_supp		= 1,
 	.default_submit_type		= TARGET_QUEUE_SUBMIT,
 	.direct_submit_supp		= 1,
 };