Add 64-bit atomic ops for armv4, only for kernel code, mostly so that we
don't need any #ifdef stuff to use atomic_load/store_64() elsewhere in
the kernel.  For armv4 the atomics are trivial to implement for kernel
code (just disable interrupts), less so for user mode, so this only has
the kernel mode implementations for now.
Use atomic_load/store_64() in the arm implementation of counter(9), and
remove the XXX comments about non-atomic access to the counters.
Don't return ZIO_PIPELINE_CONTINUE from vdev_op_io_start methods

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