dm: merge max_hw_sector

Make sure dm honours max_hw_sectors of underlying devices

  We still have no firm testing evidence in support of this patch but
  believe it may help to resolve some bug reports.  - agk

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>