xfs: create a helper to handle logging parts of rt bitmap/summary blocks

Create an explicit helper function to log parts of rt bitmap and summary
blocks.  While we're at it, fix an off-by-one error in two of the
rtbitmap logging calls that led to unnecessarily large log items but was
otherwise benign.

Note that the upcoming rtgroups patchset will add block headers to the
rtbitmap and rtsummary files.  The helpers in this and the next few
patches take a less than direct route through xfs_rbmblock_wordptr and
xfs_rsumblock_infoptr to avoid helper churn in that patchset.

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