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

[Shuah Khan]

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

test_vsyscall.c: In function =E2=80=98test_process_vm_readv=E2=80=99:
test_vsyscall.c:500:22: warning: =E2=80=98__builtin_memcmp_eq=E2=80=99 spec=
ified bound
4096 exceeds source size 0 [-Wstringop-overread]
  500 |                 if (!memcmp(buf, (const void
*)0xffffffffff600000, 4096)) {
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~=
~~~~~

Signed-off-by: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>

- Naresh