[$] Lazy file reflink

<p>Amir Goldstein has a use case for a feature that could be called a "lazyfile reflink", he said, though it might also be described as "VFS-levelsnapshots". He went through the use case, looking for suggestions, in asession at the 2019 Linux Storage, Filesystem, and Memory-Management Summit(LSFMM). He has already implemented parts of the solution, but would liketo get something upstream, which would mean shifting from thestacked-filesystem approach he has taken so far.