Avoid panic when adjusting priority of a read in the face of an IO error

Avoid panic when adjusting priority of a read in the face of an IO error

PR:		231516
Reported by:	sbruno
Approved by:	re (rgrimes)
Obtained from:	ZFS-on-Linux
X-MFC-with:	334844
Sponsored by:	Klara Systems

MFV/ZoL:	Fix zio->io_priority failed (7 < 6) assert

commit c26cf0966d131b722c32f8ccecfe5791a789d975
Author: Tony Hutter <hutter2@llnl.gov>
Date:   Tue May 29 18:13:48 2018 -0700

  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>