[LSF/MM TOPIC] block: extend generic biosets to allow per-device frontpad

[Mike Snitzer <snitzer@redhat.com>]

I'd like to enable bio-based DM to _not_ need to clone bios.  But to do
so each bio-based DM target's required per-bio-data would need to be
provided by upper layer biosets (as opposed to the bioset DM currently
creates).

So my thinking is that all system-level biosets (e.g. fs_bio_set,
blkdev_dio_pool) would redirect to a device specific variant bioset IFF
the underlying device advertises the need for a specific per-bio-data
payload to be provided.

I know this _could_ become a rathole but I'd like to avoid reverting DM
back to the days of having to worry about managing mempools for the
purpose of per-io allocations.  I've grown spoiled by the performance
and elegance that comes with having the bio and per-bio-data allocated
from the same bioset.

Thoughts?
Mike