block: max hardware sectors limit wrapper

Implement blk_limits_max_hw_sectors() and make
blk_queue_max_hw_sectors() a wrapper around it.

DM needs this to avoid setting queue_limits' max_hw_sectors and
max_sectors directly.  dm_set_device_limits() now leverages
blk_limits_max_hw_sectors() logic to establish the appropriate
max_hw_sectors minimum (PAGE_SIZE).  Fixes issue where DM was
incorrectly setting max_sectors rather than max_hw_sectors (which
caused dm_merge_bvec()'s max_hw_sectors check to be ineffective).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@kernel.org
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>