vdev_mirror: load balancing fixes

vdev_queue:
- Track the last position of each vdev, including the io size,
  in order to detect linear access of the following zio.
- Remove duplicate `vq_lastoffset`

vdev_mirror:
- Correctly calculate the zio offset (signedness issue)
- Deprecate `vdev_queue_register_lastoffset()`
- Add `VDEV_LABEL_START_SIZE` to zio offset of leaf vdevs

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Closes #6461

include/sys/vdev.h

@@ -125,8 +125,7 @@ extern zio_t *vdev_queue_io(zio_t *zio);
 extern void vdev_queue_io_done(zio_t *zio);
 
 extern int vdev_queue_length(vdev_t *vd);
-extern uint64_t vdev_queue_lastoffset(vdev_t *vd);
-extern void vdev_queue_register_lastoffset(vdev_t *vd, zio_t *zio);
+extern uint64_t vdev_queue_last_offset(vdev_t *vd);
 
 extern void vdev_config_dirty(vdev_t *vd);
 extern void vdev_config_clean(vdev_t *vd);

include/sys/vdev_impl.h

@@ -127,7 +127,6 @@ struct vdev_queue {
 	hrtime_t	vq_io_delta_ts;
 	zio_t		vq_io_search; /* used as local for stack reduction */
 	kmutex_t	vq_lock;
-	uint64_t	vq_lastoffset;
 };
 
 /*