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: Huapeng Zhou <hzhou@fb.com>
Tested-by: Feixiong Zhang <feixiong@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>