xfs: close some gaps in scrub checking

This patchset contains a bunch of small fixes and enhancements to the
online scrub and repair code.  First, we enhance the rmapbt key
comparison functions to include the attr and bmbt bits, since those
actually /are/ part of the owner information, and online fsck needs it
to be able to validate rmapbt records completely.

Fix a deficiency in the scrubbers where we don't quite properly detect
gaps in records.  Also, enhance the fscounters scrubber to check the
free rt extent count, look for oversized rmaps when checking the bmap,
and verify the non-shared status of extents not explicitly covered by
refcount records.

Also fix some bugs with DONTCACHE inodes being dropped during a scrub
scan, which is a no-no.
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