Counting I/O errors in /sys/block/<dev>/stat

Dear colleagues,

The attached patch keeps a count of block device I/O errors -- any error
event that generates a klog message in blk_update_request -- and reports
the count as a 12th field in /sys/block/<dev>/stat. That allows, e.g.,
monitoring systems to detect and count block device errors without having
to scrape syslog.

If the patch is acceptable as is, please consider it submitted for eventual
merging into the mainline kernel. If not, I'd welcome comments, feedback,
etc.

Thanks,

Jim Shankland