block, bfq: increase time window for waker detection

Tests on slower machines showed current window to be way too
small. This commit increases it.

Tested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>