Add traffic policy for low cache available.

euleros inclusion
category: feature
bugzilla: https://bugzilla.openeuler.org/show_bug.cgi?id=327


CVE: NA

When cache available is low, bcache turn to writethrough mode. Therefore,
All write IO will be directly sent to backend device, which is usually
HDD. At same time, cache device flush dirty data to the backend device
in the bcache writeback process. So write IO from user will damage the
sequentiality of writeback. And if there is lots of IO from writeback,
user's write IO may be block. This patch add traffic policy in bcache
to solve the problem and improve the performance for bcache when cache
available is low.

Signed-off-by: qinghaixiang <xuweiqhx@163.com>
Signed-off-by: Xu Wei <xuwei56@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Reviewed-by: Li Ruilin <liruilin4@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>