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

tools/testing/selftests/bpf/netcnt_common.h

@@ -8,11 +8,11 @@
 
 /* sizeof(struct bpf_local_storage_elem):
  *
- * It really is about 128 bytes on x86_64, but allocate more to account for
- * possible layout changes, different architectures, etc.
+ * It is about 128 bytes on x86_64 and 512 bytes on s390x, but allocate more to
+ * account for possible layout changes, different architectures, etc.
  * The kernel will wrap up to PAGE_SIZE internally anyway.
  */
-#define SIZEOF_BPF_LOCAL_STORAGE_ELEM		256
+#define SIZEOF_BPF_LOCAL_STORAGE_ELEM		768
 
 /* Try to estimate kernel's BPF_LOCAL_STORAGE_MAX_VALUE_SIZE: */
 #define BPF_LOCAL_STORAGE_MAX_VALUE_SIZE	(0xFFFF - \