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: 076a95f5 ("selftests/bpf: Add bpf_snprintf_btf helper tests")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210227051726.121256-1-iii@linux.ibm.com

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

@@ -16,6 +16,13 @@ bool skip = false;
 #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
@@ -113,11 +120,11 @@ int BPF_PROG(trace_netif_receive_skb, struct sk_buff *skb)
 	}
 
 	/* 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;
 	}