selftests/bpf: Fix test_xdp_adjust_tail_grow2 on s390x

s390x cache line size is 256 bytes, so skb_shared_info must be aligned
on a much larger boundary than for x86.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-17-iii@linux.ibm.com


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