Fix vdev_probe() call outside SCL_STATE_ALL lock

When an IO fails then zio_vdev_io_done() can call vdev_probe()
to determine the health of the vdev.  This is safe as long as
the original zio was submitted with zio_wait() and holds the
SCL_STATE_ALL lock over the operation.

If zio_no_wait() was used then the done callback will submit
the probe IO outside the SCL_STATE_ALL lock and hit this
ASSERT in zio_create()

  ASSERT(!vd || spa_config_held(spa, SCL_STATE_ALL, RW_READER));

Resolve the issue by only allowing vdev_probe() to be called
when there's a waiter indicating the caller is using zio_wait().
This assumes that caller is still holding SCL_STATE_ALL.

This issue isn't MMP specific but was surfaced when testing.
Without this patch it can be reproduced by running:

  zpool set multihost on <pool>
  zinject -d <vdev> -e io -T write -f 50 <pool> -L uber

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@intel.com>
Closes #745
Closes #6279

module/zfs/zio.c

@@ -3325,7 +3325,7 @@ zio_vdev_io_done(zio_t *zio)
 
 	ops->vdev_op_io_done(zio);
 
-	if (unexpected_error)
+	if (unexpected_error && zio->io_waiter != NULL)
 		VERIFY(vdev_probe(vd, zio) == NULL);
 
 	return (ZIO_PIPELINE_CONTINUE);