Fix KFUNC_PROBE return value

The KFUNC_PROBE macro is using "void" as return type, this is causing problems
in some tools that have a filtering enable that returns 0.

Reproducer: (Notice that it requires BTF support)

```
$ python opensnoop.py --pid 5
/virtual/main.c:33:21: error: void function '____kretfunc__do_sys_open' should not return a value [-Wreturn-type]
    if (pid != 5) { return 0; }
                    ^      ~
1 error generated.
...
```

Signed-off-by: Mauricio Vásquez <[email protected]>

src/cc/export/helpers.h

@@ -1012,7 +1012,7 @@ int raw_tracepoint__##event(struct bpf_raw_tracepoint_args *ctx)
 #define BPF_PROG(name, args...) \
 int name(unsigned long long *ctx); \
 __attribute__((always_inline)) \
-static void ____##name(unsigned long long *ctx, ##args); \
+static int ____##name(unsigned long long *ctx, ##args);  \
 int name(unsigned long long *ctx) \
 { \
  _Pragma("GCC diagnostic push") \
@@ -1021,7 +1021,7 @@ int name(unsigned long long *ctx) \
  _Pragma("GCC diagnostic pop") \
  return 0; \
 } \
-static void ____##name(unsigned long long *ctx, ##args)
+static int ____##name(unsigned long long *ctx, ##args)
 
 #define KFUNC_PROBE(event, args...) \
  BPF_PROG(kfunc__ ## event, args)

tools/klockstat.py

@@ -352,17 +352,17 @@ def stack_id_err(stack_id):
 program_kfunc = """
 KFUNC_PROBE(mutex_unlock, void *lock)
 {
- do_mutex_unlock_enter();
+ return do_mutex_unlock_enter();
 }
 
 KRETFUNC_PROBE(mutex_lock, void *lock, int ret)
 {
- do_mutex_lock_return();
+ return do_mutex_lock_return();
 }
 
 KFUNC_PROBE(mutex_lock, void *lock)
 {
- do_mutex_lock_enter(ctx, 3);
+ return do_mutex_lock_enter(ctx, 3);
 }
 
 """

tools/opensnoop.py

@@ -197,6 +197,8 @@
  data.ret = ret;
 
  events.perf_submit(ctx, &data, sizeof(data));
+
+ return 0:
 }
 """