selftests/bpf: Fix xdp_do_redirect on s390x

s390x cache line size is 256 bytes, so skb_shared_info must be aligned
on a much larger boundary than for x86. This makes the maximum packet
size smaller.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-11-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>