zio_free_control is needed by external buffers.

zio_alloc_control will be needed by external triggers that don't rely
on zio_fire_trigger(). It will also be needed by in-kernel sources
of output data.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>