cleanup bio page releasing

Christoph Hellwig
2019-06-26T13:49:33Z

Hi Jens,

this series cleans up the various direct I/O and pass through
routines by switching them over to common bio helpers.

The last patch just unconditionally applies the no page ref
behavior for bvec iters. I looked at all the callers, and
there is none that drops the pre-required references before
completing the request.