xfs: create deferred log items for file mapping exchanges

xfs: create deferred log items for file mapping exchanges

Source kernel commit: 966ceafc7a437105ecfe1cadb3747b2965a260ca

Now that we've created the skeleton of a log intent item to track and
restart file mapping exchange operations, add the upper level logic to
log.  This builds on the existing bmap update intent items that have
been around for a while now.

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