bcache: provide a switch to bypass all IO requests

euleros inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4LOJ6


CVE: NA

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provide a switch named read_bypass. If enbale, all IO requests will
bypass the cache. This option could be useful when we enable userspace
prefetch and the cache device is low capacity.

Signed-off-by: Li Ruilin <liruilin4@huawei.com>
Reviewed-by: Luan Jianhai <luanjianhai@huawei.com>
Reviewed-by: Peng Junyi <pengjunyi1@huawei.com>
Acked-by: Xie Xiuqi <xiexiuqi@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Reviewed-by: Guangxing Deng <dengguangxing@huawei.com>
Reviewed-by: chao song <chao.song@huawei.com>
Reviewed-by: chao song <chao.song@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>