This prevents recursion of vdev_queue_io_done as per r265321 but
using a different method as recommended on the openzfs list.
We now use zio_interrupt(zio) and return ZIO_PIPELINE_STOP instead
of returning ZIO_PIPELINE_CONTINUE from vdev_*_io_start methods.
zio_vdev_io_start now ASSERTS the that vdev_op_io_start returns
ZIO_PIPELINE_STOP to ensure future changes don't reintroduce
ZIO_PIPELINE_CONTINUE returns.
Cleanup flow in vdev_geom_io_start while I'm here.
Also fix some cases not using SET_ERROR(..)
MFC after: 2 weeks
X-MFC-With: r265321
6a369c018c