[$] Lazy file reflink

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