Use specified buffer_pattern (if given) for all io_u fills

Use specified buffer_pattern (if given) for all io_u fills

For compression, we use a fixed '0' pattern. But if the user
specified a pattern to use in the job file, then we should
use that instead. It could slightly skew the compression ratio
for long patterns, but that is to be expected.

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