We cannot free disk blocks until I/O against them has completed. Otherwise
the block could be reused for something else, then the old IO will scribble
on the new data.
What we _can_ do is to defer the waiting - only wait on the I/O when someone
reuses the disk blocks. So there are actually unused blocks with I/O in
flight against them.
We do that for metadata (the wait happens in unmap_underlying_metadata()) but
for file data blocks there is no mechanism in place to look them up.
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