xfs: bind together the front and back ends of the file range exchange code

xfs: bind together the front and back ends of the file range exchange code

So far, we've constructed the front end of the file range exchange code
that does all the checking; and the back end of the file mapping
exchange code that actually does the work.  Glue these two pieces
together so that we can turn on the functionality.

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