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>