[$] Atomic extent swapping for XFS

Normally, files exist in a filesystem to keep data contained within themseparated; seeing data exchanged directly between files is often a sign of filesystemcorruption. There are, however, use cases where it is desirable to be ableto perform a controlled swap of data between a pair of files. Darrick Wonghas recently posted apatch set implementing this feature for the XFS filesystem, but alsomaking it available in a general way.