xfs: track usage statistics of online fsck

Track the usage, outcomes, and run times of the online fsck code, and
report these values via debugfs. The columns in the file are:

* scrubber name

* number of scrub invocations
* clean objects found
* corruptions found
* optimizations found
* cross referencing failures
* inconsistencies found during cross referencing
* incomplete scrubs
* warnings
* number of time scrub had to retry
* cumulative amount of time spent scrubbing (microseconds)

* number of repair inovcations
* successfully repaired objects
* cumuluative amount of time spent repairing (microseconds)

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>