bpf: use the same condition in perf event set/free bpf handler

This is a cleanup such that doing the same check in
perf_event_free_bpf_prog as we already do in
perf_event_set_bpf_prog step.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>