父进程启动bthread后，fork出子进程后子进程踩内存

**Describe the bug (描述bug)**
1、父进程启动bthread
2、fork出子进程
3、子进程的Sampler踩了内存

**To Reproduce (复现方法)**
编译执行 使用valgrind可见
```C++
#include
#include
#include
#include
#include
#include
#include
using namespace std;

void child_proc();
void parent_proc();
void *empty_th(void *) { std::cout << "do nothing but start bthread worker\n"; }
int main(int argc, char *argv[]) {
gflags::ParseCommandLineFlags(&argc, &argv, true);

// preload: start bthread
bthread_t tid;
bthread_start_background(&tid, nullptr, empty_th, nullptr);
bthread_join(tid, nullptr);
std::cout << "start to fork\n";
int ret = fork();
if (ret == 0) {
child_proc();
} else if (ret > 0) {
parent_proc();
} else {
assert(false && "cannot fork");
}
return 0;
}

void child_proc() {
std::cout << "start child, pid: " << getpid() << "\n";
for (int i = 0; i < 40; i++) {
std::thread th([&] {
char mem[40960];
memset(mem, 0x7f, sizeof(mem));
while(true) sleep(3);
});
th.detach();
}
while(true) sleep(3);
}

void parent_proc() {
std::cout << "start parent, pid: " << getpid() << "\n";
while(true) sleep(3);
}

/*
* g++ -std=c++11 ./test.cc -lpthread -lbrpc -lgflags
* valgrind ./a.out
*/
```
```
(gdb) bt
#0 0x00007ff167ae948c in get_value (this=0x7ff1636628d0, this=0x7ff1636628d0) at /home/brpc-master/src/bvar/passive_status.h:140
#1 bvar::detail::ReducerSampler, unsigned long, bvar::detail::AddTo, bvar::detail::MinusFrom >::take_sample (this=0x7ff154008c90) at /home/brpc-master/src/bvar/detail/sampler.h:137
#2 0x00007ff167af5b11 in bvar::detail::SamplerCollector::run (this=this@entry=0xcf9a60) at /home/brpc-master/src/bvar/detail/sampler.cpp:173
#3 0x00007ff167af67f0 in bvar::detail::SamplerCollector::sampling_thread (arg=0xcf9a60) at /home/brpc-master/src/bvar/detail/sampler.cpp:113
#4 0x00007ff168da2864 in start_thread (arg=0x7ff142ffd700) at pthread_create.c:477
#5 0x00007ff166b23c1f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
(gdb) f 0
#0 0x00007ff167ae948c in get_value (this=0x7ff1636628d0, this=0x7ff1636628d0) at /home/brpc-master/src/bvar/passive_status.h:140
140 return (_getfn ? _getfn(_arg) : Tp());
(gdb) up
#1 bvar::detail::ReducerSampler, unsigned long, bvar::detail::AddTo, bvar::detail::MinusFrom >::take_sample (this=0x7ff154008c90) at /home/brpc-master/src/bvar/detail/sampler.h:137
137 latest.data = _reducer->get_value();
(gdb) p _reducer
$1 = (bvar::PassiveStatus *) 0x7ff1636628d0
(gdb) p *_reducer
$2 = { = {_vptr.Variable = 0x7f7f7f7f7f7f7f7f, _name = {static npos = 18446744073709551615,
_M_dataplus = {> = {<__gnu_cxx::new_allocator> = {}, },
_M_p = 0x7f7f7f7f7f7f7f7f

Reproduce the fork scenario with the supplied C++ example and Valgrind, then inspect src/bvar/passive_status.h, src/bvar/detail/sampler.h, and src/bvar/detail/sampler.cpp around the reported stack frames. Verify the suspected interaction between forked child threads and sampler state. Done means the child process can create and run its threads without invalid sampler accesses or memory corruption.

Written by the indexing model from the issue text.