Rework the I/O accounting for bio based drivers to use ->bi_bdev. This
means all drivers can now simply use bio_start_io_acct to start
accounting, and it will take partitions into account automatically. To
end I/O account either bio_end_io_acct can be used if the driver never
remaps I/O to a different device, or bio_end_io_acct_remapped if the
driver did remap the I/O.