md: expose max value of behind writes counter
Paul Clements authored

Keep track of the maximum number of concurrent write-behind requests
for an md array and exposed this number in sysfs at
   md/bitmap/max_backlog_used

Writing any value to this file will clear it.

This allows userspace to be involved in tuning bitmap/backlog.
Signed-off-by: default avatarPaul Clements <paul.clements@steeleye.com>
Signed-off-by: default avatarNeilBrown <neilb@suse.de>
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