blk-mq: fix divide by zero crash in tg_may_dispatch()

hulk inclusion
category: bugfix
bugzilla: 177149, https://gitee.com/openeuler/kernel/issues/I47R8R


CVE: NA

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If blk-throttle is enabled and io is issued before
blk_throtl_register_queue() is done. Divide by zero crash will be
triggered in tg_may_dispatch() because 'throtl_slice' is uninitialized.

Thus introduce a new falg QUEUE_FLAG_THROTL_INIT_DONE. It will be set
after blk_throtl_register_queue() is done, and will be checked before
apply any config.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>