Fix verify random bytes filling

Shawn Lewis <shawnlewis@google.com> reports:

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verify.c: fill_random_bytes() doesn't work as intended.

drand48 on linux returns a double whose value is evenly distributed between
0.0 and 0.1. This is not the same as having an 8-byte value where each bit
has the same chance of being 0 as it does of being 1.

I'd fix this but it should probably be done in a cross-platformish way and
I'm not quite sure how to do it. Maybe just use smaller randoms? Have an
os_random_uint32?

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I just got rid of os_random_double() and used os_random_long(),
truncating that to an unsigned integer.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>