selftests: x86: fix [-Wstringop-overread] warn in test_process_vm_readv()

stable inclusion
from stable-v5.10.91
commit c1e2da4b3f72a0dc312f6b35ec3d3a39777f54e2
bugzilla: 186187 https://gitee.com/openeuler/kernel/issues/I4SI2C

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c1e2da4b3f72a0dc312f6b35ec3d3a39777f54e2



--------------------------------

commit dd40f44e upstream.

Fix the following [-Wstringop-overread] by passing in the variable
instead of the value.

test_vsyscall.c: In function ‘test_process_vm_readv’:
test_vsyscall.c:500:22: warning: ‘__builtin_memcmp_eq’ specified bound 4096 exceeds source size 0 [-Wstringop-overread]
  500 |                 if (!memcmp(buf, (const void *)0xffffffffff600000, 4096)) {
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Cc: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>