xfs: enable logged file mapping exchange feature

Add the XFS_SB_FEAT_INCOMPAT_EXCHRANGE feature to the set of features
that we will permit when mounting a filesystem.  This turns on support
for the file range exchange feature.

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