xfs: remove xfs_rtb_to_rtxrem

Simplify the number of block number conversion helpers by removing
xfs_rtb_to_rtxrem.  Any recent compiler is smart enough to eliminate
the double divisions if using separate xfs_rtb_to_rtx and
xfs_rtb_to_rtxoff calls.

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>