samples: bpf: enable trace samples for s390x

The trace bpf samples do not compile on s390x because they use x86
specific fields from the "pt_regs" structure.

Fix this and access the fields via new PT_REGS macros.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/samples/bpf/tracex1_kern.c b/samples/bpf/tracex1_kern.c
index 3162046..3f450a8 100644
--- a/samples/bpf/tracex1_kern.c
+++ b/samples/bpf/tracex1_kern.c
@@ -29,7 +29,7 @@
 	int len;
 
 	/* non-portable! works for the given kernel only */
-	skb = (struct sk_buff *) ctx->di;
+	skb = (struct sk_buff *) PT_REGS_PARM1(ctx);
 
 	dev = _(skb->dev);