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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/netcnt_common.h

···
 
 /* 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 - \