Move only dirty data when gc runnning, in order to reducing write amplification.

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


CVE: NA

Bcache will move all data, including clean and dirty data, in bucket when
gc running. This will cause big write amplification, which may reduce the
cache device's life. This patch provice a switch for gc to move only dirty
data, which can reduce write amplification.

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>