block I/O accounting improvements v2

Hi Jens, they series contains various improvement for block I/O accounting. The first bunch of patches switch the bio based drivers to better accounting helpers compared to the current mess. The end contains a fix and various performanc improvements. Most of this comes from a series Konstantin sent a few weeks ago, rebased on changes that landed in your tree since and my change to always use the percpu version of the disk stats. Changes since v1: - add an ifdef CONFIG_BLOCK to work around the sad state of our headers - add reviewed-by tags to all patches