Fix ZIO reordering done by vdev_queue_io causing panics when zio_vdev_io_start
returns ZIO_PIPELINE_CONTINUE from vdev_op_io_start to zio_execute resulting
in the wrong ZIO continuing its pipeline.

This is a serious issue which could cause data loss / corruption but appears
to be limited to error handling such as when vdev_readable(vd) returns false.

MFC after:	2 days