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/tracex3_kern.c b/samples/bpf/tracex3_kern.c
index 255ff27..bf337fb 100644
--- a/samples/bpf/tracex3_kern.c
+++ b/samples/bpf/tracex3_kern.c
@@ -23,7 +23,7 @@
 SEC("kprobe/blk_mq_start_request")
 int bpf_prog1(struct pt_regs *ctx)
 {
-	long rq = ctx->di;
+	long rq = PT_REGS_PARM1(ctx);
 	u64 val = bpf_ktime_get_ns();
 
 	bpf_map_update_elem(&my_map, &rq, &val, BPF_ANY);
@@ -51,7 +51,7 @@
 SEC("kprobe/blk_update_request")
 int bpf_prog2(struct pt_regs *ctx)
 {
-	long rq = ctx->di;
+	long rq = PT_REGS_PARM1(ctx);
 	u64 *value, l, base;
 	u32 index;