iolog: propagate errors to caller

Signed-off-by: Jens Axboe <axboe@fb.com>
iolog: fix compile error for non-zlib

Signed-off-by: Jens Axboe <axboe@fb.com>
iolog: fix link without zlib

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