vhost-scsi: Allocate T10 PI structs only when enabled

T10 PI is not a widely used feature. This has us only allocate the
structs for it if the feature has been enabled. For a common small setup
where you have 1 virtqueue and 128 commands per queue, this saves:

8MB = 32 bytes per sg * 2048 entries * 128 commands

per vhost-scsi device.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20241203191705.19431-3-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

drivers/vhost/scsi.c

@@ -1651,12 +1651,14 @@ static int vhost_scsi_setup_vq_cmds(struct vhost_virtqueue *vq, int max_cmds)
 			goto out;
 		}
 
-		tv_cmd->tvc_prot_sgl = kcalloc(VHOST_SCSI_PREALLOC_PROT_SGLS,
-					       sizeof(struct scatterlist),
-					       GFP_KERNEL);
-		if (!tv_cmd->tvc_prot_sgl) {
-			pr_err("Unable to allocate tv_cmd->tvc_prot_sgl\n");
-			goto out;
+		if (vhost_has_feature(vq, VIRTIO_SCSI_F_T10_PI)) {
+			tv_cmd->tvc_prot_sgl = kcalloc(VHOST_SCSI_PREALLOC_PROT_SGLS,
+						sizeof(struct scatterlist),
+						GFP_KERNEL);
+			if (!tv_cmd->tvc_prot_sgl) {
+				pr_err("Unable to allocate tv_cmd->tvc_prot_sgl\n");
+				goto out;
+			}
 		}
 	}
 	return 0;