switch block layer polling to a bio based model v4

[Keith Busch]

On Tue, Jun 15, 2021 at 03:10:18PM +0200, Christoph Hellwig wrote:
> Chances since v3:
>  - rebased to the latests for-5.14/block tree
>  - fix the refcount logic in __blkdev_direct_IO
>  - split up a patch to make it easier to review
>  - grab a queue reference in bio_poll
>  - better document the RCU assumptions in bio_poll

It still doesn't look like a failover will work when polling through
pvsync2. We previously discussed that here:

  https://marc.info/?l=linux-block&m=162100971816071&w=2