blk-throttle: don't check whether or not lower limit is valid if...

blk-throttle: don't check whether or not lower limit is valid if CONFIG_BLK_DEV_THROTTLING_LOW is off

mainline inclusion
from mainline-v5.11-rc1
commit acaf523a
category: bugfix
bugzilla: 185789 https://gitee.com/openeuler/kernel/issues/I4DDEL
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=acaf523a7bf226b28504306c1cfee194520123b3



---------------------------

blk_throtl_update_limit_valid() will search for descendants to see if
'LIMIT_LOW' of bps/iops and READ/WRITE is nonzero. However, they're always
zero if CONFIG_BLK_DEV_THROTTLING_LOW is not set, furthermore, a lot of
time will be wasted to iterate descendants.

Thus do nothing in blk_throtl_update_limit_valid() in such situation.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Yufen Yu <yuyufen@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>

Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>