xfs_repair: check existing realtime refcountbt entries against observed refcounts

xfs_repair: check existing realtime refcountbt entries against observed refcounts

Once we've finished collecting reverse mapping observations from the
metadata scan, check those observations against the realtime refcount
btree (particularly if we're in -n mode) to detect rtrefcountbt
problems.

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