xfs: add usage counters for scrub [v26.1]

This series introduces simple usage and performance counters for the
online fsck subsystem.  The goal here is to enable developers and
sysadmins to look at summary counts of how many objects were checked and
repaired; what the outcomes were; and how much time the kernel has spent
on these operations.  The counter file is exposed in debugfs because
that's easier than cramming it into the device model, and debugfs
doesn't have rules against complex file contents, unlike sysfs.

This has been running on the djcloud for months with no problems.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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