In 4.4, truncate (which is now misnamed) will increase a file's
size.  (It would be nice to get the system call renamed ... but then,
O_CREAT is still spelled without its final E. :-) )

The code for extending a file is tricky.  It took a couple of rounds
(with some nasty little fs problems) to get this to work right.  If
you extend a block in place, you must zero out the new parts; if you
grow a file beyond the `small file' size, you must extend its final
fragment, if any, to a full block.

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