block I/O accounting improvements

[Christoph Hellwig]

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.