xfs_repair: improve rtbitmap discrepancy reporting

xfs_repair: improve rtbitmap discrepancy reporting

Improve the reporting of discrepancies in the realtime bitmap and
summary files by creating a separate helper function that will pinpoint
the exact (word) locations of mismatches.  This will help developers to
diagnose problems with the rtgroups feature and users to figure out
exactly what's bad in a filesystem.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>