samples/bpf: xdp_monitor first 8 bytes are not accessible by bpf

The first 8 bytes of the tracepoint context struct are not accessible
by the bpf code.  This is a choice that dates back to the original
inclusion of this code.

See explaination in:
 commit 98b5c2c6 ("perf, bpf: allow bpf programs attach to tracepoints")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/xdp_monitor_kern.c

@@ -17,11 +17,7 @@ struct bpf_map_def SEC("maps") redirect_err_cnt = {
  * Code in:                kernel/include/trace/events/xdp.h
  */
 struct xdp_redirect_ctx {
-	unsigned short common_type;	//	offset:0;  size:2; signed:0;
-	unsigned char common_flags;	//	offset:2;  size:1; signed:0;
-	unsigned char common_preempt_count;//	offset:3;  size:1; signed:0;
-	int common_pid;			//	offset:4;  size:4; signed:1;
-
+	u64 __pad;		// First 8 bytes are not accessible by bpf code
 	int prog_id;		//	offset:8;  size:4; signed:1;
 	u32 act;		//	offset:12  size:4; signed:0;
 	int ifindex;		//	offset:16  size:4; signed:1;