bloom: Use truncate not writing zeros in create

bloom: Use truncate not writing zeros in create

This lets us test more of bloom's code without writing gigabyte(s) of
zeros to disk.  As noted in the NOTE: this works on all of the common
modern unixes that I checked, but may need special handling on other
systems.

Signed-off-by: Brandon Low <lostlogic@lostlogicx.com>