queue_sector_alignment_offset returned the wrong value which caused
partitions to report an incorrect alignment_offset.  Since offset
alignment calculation is needed several places it has been split into a
separate helper function.  The topology stacking function has been
updated accordingly.

Furthermore, comments have been added to clarify how the stacking
function works.
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>
Tested-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>