bcache: always record start time of a sample

euleros/rtos inclusion
category: bugfix
bugzilla: https://bugzilla.openeuler.org/show_bug.cgi?id=26


CVE: NA

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Before this patch bypassed I/O request will not record start time,
therefore when calculating latency data, start time will be calculated
wrongly. This patch makes start time always be recorded to fix this.

Signed-off-by: Li Ruilin <liruilin4@huawei.com>
Reviewed-by: Song Chao <chao.song@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>