This feels like it's heading in the wrong direction, as it just adds
more hacks.
IMHO the right thing is to:
a) document current assumptions and tell users very clearly to use
dm-thin to get the expected semantics
b) if we have a use for it, provide an option to use write zeroes
instead that can be used on any device
I'm happy to do that work, but until then I'd suggest to skip this and
the next two patches.
