perf/bpf: Robustify perf_event_free_bpf_prog()

Ensure perf_event_free_bpf_prog() is safe to call a second time;
notably without making any references to event->pmu when there is no
prog left.

Note: perf_event_detach_bpf_prog() might leave a stale event->prog

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Ravi Bangoria <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Peter Zijlstra

kernel/events/core.c

@@ -10905,6 +10905,9 @@ int perf_event_set_bpf_prog(struct perf_event *event, struct bpf_prog *prog,
 
 void perf_event_free_bpf_prog(struct perf_event *event)
 {
+	if (!event->prog)
+		return;
+
 	if (!perf_event_is_tracing(event)) {
 		perf_event_free_bpf_handler(event);
 		return;