bcache: Add a sample of userspace prefetch client

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


CVE: NA

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Add a sample for new prefetch frame on bcache. As a sample, the
program just reads every read request received by bcache device
and send a prefetch request for each read request. The length of
the prefetch request is equal as the read request and the position
of the prefetch request is follow the read request.

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>