Switch zram to use the nicer bio accounting helpers, and as part of that
ensure each bio is counted as a single I/O request.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Jens Axboe <axboe@kernel.dk>