fs/stat: make tmp variable 16-bytes aligned in copy stat

hulk inclusion
category: performance
bugzilla: https://gitee.com/openeuler/kernel/issues/I7F4XV



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The tmp variable is used to copy_to_user(), it has better
performance if the address accesseed by ldp instruction
is 16 bytes aligned on arm64.

The performance of nginx test is improved after this patch:
http "Connection: close"	1.11%
http "Connection: keep-alive"	2.11%
https "Connection: close"	1.56%
https "Connection: keep-alive"	0.18%

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>