selftests/bpf: Fix xdp_do_redirect on s390x

s390x cache line size is 256 bytes, so skb_shared_info must be aligned
on a much larger boundary than for x86. This makes the maximum packet
size smaller.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-11-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c

@@ -65,7 +65,11 @@ static int attach_tc_prog(struct bpf_tc_hook *hook, int fd)
 /* The maximum permissible size is: PAGE_SIZE - sizeof(struct xdp_page_head) -
  * sizeof(struct skb_shared_info) - XDP_PACKET_HEADROOM = 3368 bytes
  */
+#if defined(__s390x__)
+#define MAX_PKT_SIZE 3176
+#else
 #define MAX_PKT_SIZE 3368
+#endif
 static void test_max_pkt_size(int fd)
 {
 	char data[MAX_PKT_SIZE + 1] = {};