xfs: calculate RT bitmap and summary blocks based on sb_rextents

xfs: calculate RT bitmap and summary blocks based on sb_rextents

Use the on-disk rextents to calculate the bitmap and summary blocks
instead of the calculated one so that we can refactor the helpers for
calculating them.

As the RT bitmap and summary scrubbers already check that sb_rextents
match the block count this does not change coverage of the scrubber.

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