xfs: add a xfs_rtrmap_highest_rgbno helper

Add a helper to find the last offset mapped in the rtrmap.  This will be
used by the zoned code to find out where to start writing again on
conventional devices without hardware zone support.

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