common: fix races when visiting a TrackedOp.

common: fix races when visiting a TrackedOp.

Before the commit following race can happen:
  ```
  A           : OpTracker::visit_ops_in_flight(..., callable leaking TrackedOpRef outside)
  A           : Mutex::Locker::Locker(sdata->ops_in_flight_lock_sharded)
  A with lock : (nref > 0) == true

  B           : TrackedOp::put(), nref := 0 // updating the counter is done without the lock
  B           : OpTracker::unregister_inflight_op()
  B           : Mutex::Locker::Locker(sdata->ops_in_flight_lock_sharded)

  A with lock : visit() -> TrackedOp::get(), nref := 1
  A with lock : Mutex::Locker::~Locker()

  B with lock : boost::intrusive::list::iterator_to(op)
  B with lock : boost::intrusive::list::erase(iter)
  B with lock : Mutex::Locker::~Locker()

  A           : TrackedOp::put(), nref := 0
  A           : OpTracker::unregister_inflight_op()
  A           : Mutex::Locker::Locker(sdata->ops_in_flight_lock_sharded)
  A with lock : boost::intrusive::list::iterator_to(op) // oops as op doesn't belong to the list anymore
  ```

Fixes: https://tracker.ceph.com/issues/24664
Related-To: https://tracker.ceph.com/issues/24037
Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>