packetpool: improve thread safety

packetpool: improve thread safety

 lock_acquire: Calling pthread_mutex_lock acquires lock PktPoolLockedStack_.mutex.
 87        SCMutexLock(&my_pool->return_stack.mutex);

CID 1554228: (#1 of 1): Indefinite wait (BAD_CHECK_OF_WAIT_COND)
dead_wait: A wait is performed without ensuring that the condition is not already satisfied while holding lock PktPoolLockedStack_.mutex. This can cause a deadlock if the notification happens before the lock is acquired.
      Acquire the lock, then check the wait condition in a loop, without releasing with the lock before the wait. This will prevent deadlocks and failed conditions from spurious wakeups.