selftests/bpf: Fix xdp_synproxy/tc on s390x

Use the correct datatype for the values map values; currently the test
works by accident, since on little-endian machines it is sometimes
acceptable to access u64 as u32.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-20-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>