bpf: Fix bpf_xdp_event_output

Fix a typo. xdp->data instead of xdp should be copied to the perf-event's
dst_buff.

Fixes: 4de16969 ("bpf: enable event output helper also for xdp types")
Reported-by: NHuapeng Zhou <hzhou@fb.com>
Tested-by: NFeixiong Zhang <feixiong@fb.com>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NMartin KaFai Lau <kafai@fb.com>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -2584,8 +2584,8 @@ BPF_CALL_5(bpf_xdp_event_output, struct xdp_buff *, xdp, struct bpf_map *, map,
 	if (unlikely(xdp_size > (unsigned long)(xdp->data_end - xdp->data)))
 		return -EFAULT;
 
-	return bpf_event_output(map, flags, meta, meta_size, xdp, xdp_size,
-				bpf_xdp_copy);
+	return bpf_event_output(map, flags, meta, meta_size, xdp->data,
+				xdp_size, bpf_xdp_copy);
 }
 
 static const struct bpf_func_proto bpf_xdp_event_output_proto = {