Block: blk-throttle: set low_valid immediately once one cgroup has io.low configured

Once one cgroup has io.low configured, @low_valid becomes true and other
cgroups won't switch it back whatsoever.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>