selftests: sync: Fix cast warnings on arm

Fix warnings on __u64 and pointer translation on arm and
other 32bit architectures. Since the pointer is 32bits on
those archs, we should not directly cast those types.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Emilio López <emilio.lopez@collabora.co.uk>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>