scsi: virtio_scsi: Rescan the entire target on transport reset when LUN is 0

VirtIO 1.0 spec says:

    The removed and rescan events ... when sent for LUN 0, they MAY
    apply to the entire target so the driver can ask the initiator
    to rescan the target to detect this.

This change introduces the behaviour described above by scanning the entire
SCSI target when LUN is set to 0. This is both a functional and a
performance fix. It aligns the driver with the spec and allows control
planes to hotplug targets with large numbers of LUNs without having to
request a RESCAN for each one of them.

Link: https://lore.kernel.org/r/CY4PR02MB33354370E0A81E75DD9DFE74FB520@CY4PR02MB3335.namprd02.prod.outlook.comSuggested-by: Felipe Franciosi <felipe@nutanix.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Matej Genci <matej.genci@nutanix.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/virtio_scsi.c

@@ -284,7 +284,12 @@ static void virtscsi_handle_transport_reset(struct virtio_scsi *vscsi,
 
 	switch (virtio32_to_cpu(vscsi->vdev, event->reason)) {
 	case VIRTIO_SCSI_EVT_RESET_RESCAN:
-		scsi_add_device(shost, 0, target, lun);
+		if (lun == 0) {
+			scsi_scan_target(&shost->shost_gendev, 0, target,
+					 SCAN_WILD_CARD, SCSI_SCAN_INITIAL);
+		} else {
+			scsi_add_device(shost, 0, target, lun);
+		}
 		break;
 	case VIRTIO_SCSI_EVT_RESET_REMOVED:
 		sdev = scsi_device_lookup(shost, 0, target, lun);