dm log writes: fix >512b sectorsize support

512b sectors vs device's physical sectorsize was not maintained
consistently and as such the support for >512b sector devices has bugs.
The log metadata expects native sectorsize but 512b sectors were being
stored.  Also, device's sectorsize was assumed when assigning the
bi_sector for blocks that were being logged.

Fix this up by adding two helpers to convert between bio and dev
sectors, and use these in the appropriate places to fix the problem and
make it clear which units go where.  Doing so allows dm-log-writes use
with 4k devices.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>