backend: move iolog compression init before CPU affinity settings

Previously we did this after we had set CPU affinity for the IO
worker. But if we do that, then we limit the log compression
to the exact CPUs that we don't want it to run on.

So let's do this earlier, which means the compression threads can
run anyway. We'll trust the OS scheduler to keep them out of our
way.

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