xfs: create deferred log items for file mapping exchanges

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
commit intent items and turn them into concrete work recorded in the
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>