Use a zio flag to prevent recursion of vdev_queue_io_done which can
cause stack overflow for IO's which return ZIO_PIPELINE_CONTINUE
from the zio_vdev_io_start stage and hence don't suspend and complete
in a different thread.

This prevents double fault panic on slow machines running ZFS on
GELI volumes which return EOPNOTSUPP directly to BIO_DELETE requests.

MFC after:	1 month
X-MFC-With:	r265152