client: asynchronous fsync can decrement request ref twice

After the asynchronous execution context is woken up when waiting
for Fb caps reference to be released causing the clien to crash
as per:

```
0x00007f3115b2452c in __pthread_kill_implementation () from /lib64/libc.so.6
0x00007f3115ad7686 in raise () from /lib64/libc.so.6
0x00007f3115ac1833 in abort () from /lib64/libc.so.6
0x00007f3113375d0a in ceph::__ceph_assert_fail (assertion=<optimized out>, file=<optimized out>, line=<optimized out>, func=<optimized out>) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/common/assert.cc:74
0x00007f3113375e6f in ceph::__ceph_assert_fail (ctx=...) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/common/assert.cc:79
0x00007f311237db1d in xlist<MetaRequest*>::item::~item (this=<optimized out>, this=<optimized out>) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/include/xlist.h:31
MetaRequest::~MetaRequest (this=<optimized out>, this=<optimized out>) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/client/MetaRequest.cc:65
Client::put_request (this=0x564b491726c0, request=0x7f301c0165c0) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/client/Client.cc:2140
0x00007f31123c88ad in Client::C_nonblocking_fsync_state::advance (this=0x7f307002e9f0) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/client/Client.cc:11905
0x00007f3112331ccd in Context::complete (this=0x7f3070009250, r=<optimized out>) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/include/Context.h:99
0x00007f311246a964 in Client::signal_context_list(std::__cxx11::list<Context*, std::allocator<Context*> >&) [clone .constprop.0] (ls=std::__cxx11::list = {...}, this=<optimized out>)
    at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/client/Client.cc:4257
0x00007f3112395f45 in Client::put_cap_ref (this=0x564b491726c0, in=0x7f306807be90, cap=<optimized out>) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/client/Client.cc:3611
0x00007f31123331f3 in Client::C_Write_Finisher::finish_io (r=0, this=0x7f30240442d0) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/client/Client.cc:11381
Client::CWF_iofinish::finish (this=<optimized out>, r=0) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/client/Client.h:1481
0x00007f3112331ccd in Context::complete (this=0x7f302401afd0, r=<optimized out>) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/include/Context.h:99
0x00007f31123c5242 in Client::C_Lock_Client_Finisher::finish (this=0x7f302403c9d0, r=0) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/client/Client.cc:11372
0x00007f3112331ccd in Context::complete (this=0x7f302403c9d0, r=<optimized out>) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/include/Context.h:99
0x00007f31134374ad in Finisher::finisher_thread_entry (this=0x564b491730b0) at /usr/src/debug/ceph-19.2.0-124.el9cp.x86_64/src/common/Finisher.cc:72
0x00007f3115b227e2 in start_thread () from /lib64/libc.so.6
0x00007f3115ba7800 in clone3 () from /lib64/libc.so.6
0x0000000000000000 in ?? ()
```

Fixes: http://tracker.ceph.com/issues/71510
Signed-off-by: Venky Shankar <[email protected]>

Author: vshankar

src/client/Client.cc

@@ -12137,6 +12137,7 @@ void Client::C_nonblocking_fsync_state::advance()
 
     if (waitfor_safe) {
       clnt->put_request(req);
+      waitfor_safe = false;
     }
 
     if (flush_wait && !flush_completed) {