block: don't use cmpxchg64() on 32-bit platform

hulk inclusion
category: bugfix
bugzilla: 186921
CVE: NA

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Some 32-bit platform doesn't support cmpxchg64(), using it in generic
code will cause compile error.

Fixes: 4c8f034bf1e6 ("[Huawei] block: update nsecs[] in part_stat_show() and diskstats_show()")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>