bcache: add a framework to perform prefetch

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


CVE: NA

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Add a framwork to transform io informations to userspace client and
process prefetch request sent by userspace client. Create a char device
namede "acache" for connecting between kernelspace and userspace.
Save informations of all io requests into a buffer and pass them to
client when client reads from the device.

The prefetch request could be treated as normal io request. As deference,
those requests have no need return data back to userspace, and they
should not append readahead part.

Add two parameters. acache_dev_size is for controlling size of buffer
to save io informations. acache_prefetch_workers is for controlling
max threads to process prefetch requests.

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>