t5318: replace use of /dev/zero with generate_zero_bytes

t5318: replace use of /dev/zero with generate_zero_bytes

There are platforms (e.g. NonStop) that lack /dev/zero; use the
generate_zero_bytes helper we just introduced to append stream
of NULs at the end of the file.

The original, even though it uses "dd seek=... count=..." to make it
look like it is overwriting the middle part of an existing file, has
truncated the file before this step with another use of "dd", which
may make it tricky to see why this rewrite is a correct one.

Signed-off-by: Randall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>