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

hulk inclusion
category: bugfix
bugzilla: 46357
CVE: NA

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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>
Reviewed-by: Tao Hou <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>