selftests/bpf: fix test_reuseport_array on s390

Fix endianness issue: passing a pointer to 64-bit fd as a 32-bit key
does not work on big-endian architectures. So cast fd to 32-bits when
necessary.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>