Write fast version of grow_file
Write fast version of grow_file

Grow if needed, then zero new bytes with chunked [bf_update_range oldlen
($ 0)] with appropriately sized ($ 0).

New function has the same signature and should satisfy the same spec!

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl