xfs: more scrub fixes

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.

Also included are some minor fixes to the scrub code to make sure that
we report the scrub state completely, tighten some of the checks to use
precomputed values instead of absolute maximum.  Finally, there are some
small code cleanups.