selftests/bpf: Increase SIZEOF_BPF_LOCAL_STORAGE_ELEM on s390x

selftests/bpf: Increase SIZEOF_BPF_LOCAL_STORAGE_ELEM on s390x

sizeof(struct bpf_local_storage_elem) is 512 on s390x:

    struct bpf_local_storage_elem {
            struct hlist_node          map_node;             /*     0    16 */
            struct hlist_node          snode;                /*    16    16 */
            struct bpf_local_storage * local_storage;        /*    32     8 */
            struct callback_head       rcu __attribute__((__aligned__(8))); /*    40    16 */

            /* XXX 200 bytes hole, try to pack */

            /* --- cacheline 1 boundary (256 bytes) --- */
            struct bpf_local_storage_data sdata __attribute__((__aligned__(256))); /*   256     8 */

            /* size: 512, cachelines: 2, members: 5 */
            /* sum members: 64, holes: 1, sum holes: 200 */
            /* padding: 248 */
            /* forced alignments: 2, forced holes: 1, sum forced holes: 200 */
    } __attribute__((__aligned__(256)));

As the existing comment suggests, use a larger number in order to be
future-proof.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-14-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>