xfs: create a helper to convert rtextents to rtblocks

xfs: create a helper to convert rtextents to rtblocks

Create a helper to convert a realtime extent to a realtime block.  Later
on we'll change the helper to use bit shifts when possible.

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