scsi: virtio: virtio_scsi: Set can_queue to the length of the virtqueue.

Since switching to blk-mq as the default in commit 5c279bd9e406
("scsi: default to scsi-mq"), virtio-scsi LUNs consume about 10x as
much kernel memory.

qemu currently allocates a fixed 128 entry virtqueue. can_queue
currently is set to 1024. But with indirect descriptors, each command
in the queue takes 1 virtqueue entry, so the number of commands which
can be queued is equal to the length of the virtqueue.

Note I intend to send a patch to qemu to allow the virtqueue size to be
configured from the qemu command line.

Signed-off-by: Richard W.M. Jones <[e-mail]>
Acked-by: Paolo Bonzini <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/virtio_scsi.c

···
 	.eh_timed_out = virtscsi_eh_timed_out,
 	.slave_alloc = virtscsi_device_alloc,
 
-	.can_queue = 1024,
 	.dma_boundary = UINT_MAX,
 	.use_clustering = ENABLE_CLUSTERING,
 	.target_alloc = virtscsi_target_alloc,
···
 	.eh_timed_out = virtscsi_eh_timed_out,
 	.slave_alloc = virtscsi_device_alloc,
 
-	.can_queue = 1024,
 	.dma_boundary = UINT_MAX,
 	.use_clustering = ENABLE_CLUSTERING,
 	.target_alloc = virtscsi_target_alloc,
···
 	err = virtscsi_init(vdev, vscsi);
 	if (err)
 		goto virtscsi_init_failed;
+
+	shost->can_queue = virtqueue_get_vring_size(vscsi->req_vqs[0].vq);
 
 	cmd_per_lun = virtscsi_config_get(vdev, cmd_per_lun) ?: 1;
 	shost->cmd_per_lun = min_t(u32, cmd_per_lun, shost->can_queue);