Fix zio->io_priority failed (7 < 6) assert

This fixes an assert in vdev_queue_change_io_priority():

  VERIFY3(zio->io_priority < ZIO_PRIORITY_NUM_QUEUEABLE) failed (7 < 6)
  PANIC at vdev_queue.c:832:vdev_queue_change_io_priority()

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #7566
Closes #7542