selftests/bpf: Fix cgrp_local_storage on s390x

Sync the definition of socket_cookie between the eBPF program and the
test. Currently the test works by accident, since on little-endian it
is sometimes acceptable to access u64 as u32.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-12-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/cgrp_local_storage.c

@@ -16,7 +16,7 @@
 
 struct socket_cookie {
 	__u64 cookie_key;
-	__u32 cookie_value;
+	__u64 cookie_value;
 };
 
 static void test_tp_btf(int cgroup_fd)