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

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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