fix and cleanup discard_alignment handling

[Martin K. Petersen]

Christoph,

> the somewhat confusing name of the discard_alignment queue limit, that
> really is an offset for the discard granularity mislead a lot of
> driver authors to set it to an incorrect value.  This series tries to
> fix up all these cases.

Not sure how I ended up with "discard_alignment" when I called the
corresponding I/O parameter "alignment_offset".

Anyway. All this looks good to me.

Reviewed-by: Martin K. Petersen <martin.petersen at oracle.com>

-- 
Martin K. Petersen	Oracle Linux Engineering