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.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/xdp_synproxy_kern.c

@@ -310,7 +310,7 @@ static __always_inline void values_get_tcpipopts(__u16 *mss, __u8 *wscale,
 static __always_inline void values_inc_synacks(void)
 {
 	__u32 key = 1;
-	__u32 *value;
+	__u64 *value;
 
 	value = bpf_map_lookup_elem(&values, &key);
 	if (value)