block: only use cmpxchg64 in 64bit platform

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I65K8D


CVE: NA

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Use cmpxchg64() in some 32bit platform will cause compile error, because
it might not be implemented, for example PCC32. Hence only compile it in
64bit platform.

Fixes: 82327165 ("blk-mq: don't access request_wrapper if request is not allocated from block layer")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>