selftests/bpf: Use the last page in test_snprintf_btf on s390

test_snprintf_btf fails on s390, because NULL points to a readable
struct lowcore there. Fix by using the last page instead.

Error message example:

printing fffffffffffff000 should generate error, got (361)

Fixes: 076a95f5aff2 ("selftests/bpf: Add bpf_snprintf_btf helper tests")
Signed-off-by: Ilya Leoshkevich <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Heiko Carstens <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/netif_receive_skb.c

···
 #define STRSIZE			2048
 #define EXPECTED_STRSIZE	256
 
+#if defined(bpf_target_s390)
+/* NULL points to a readable struct lowcore on s390, so take the last page */
+#define BADPTR			((void *)0xFFFFFFFFFFFFF000ULL)
+#else
+#define BADPTR			0
+#endif
+
 #ifndef ARRAY_SIZE
 #define ARRAY_SIZE(x)	(sizeof(x) / sizeof((x)[0]))
 #endif
···
 	}
 
 	/* Check invalid ptr value */
-	p.ptr = 0;
+	p.ptr = BADPTR;
 	__ret = bpf_snprintf_btf(str, STRSIZE, &p, sizeof(p), 0);
 	if (__ret >= 0) {
-		bpf_printk("printing NULL should generate error, got (%d)",
-			   __ret);
+		bpf_printk("printing %llx should generate error, got (%d)",
+			   (unsigned long long)BADPTR, __ret);
 		ret = -ERANGE;
 	}