Add support for different weights for reads and writes with bssplit

You can now do:

	bssplit=4k/20:8k/80,1k/50:4k/50

To have 20% 4k reads and 80% 8k reads, while having 50% 1k writes
and 50% 4k writes. This is identical to how the bs= option treats
reads and writes.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>