net_sched: act_bpf: remove unnecessary copy

Fix harmless typo and avoid unnecessary copy of empty 'prog' into
unused 'strcut tcf_bpf_cfg old'.

Fixes: f4eaed28 ("act_bpf: fix memory leaks when replacing bpf programs")
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>