thread_pool: fast-path _process_func when no removal is pending

Every task completion called _try_claim_and_remove_self, which
acquires _scaling_mutex and _threads_mutex even when _remove_count
is zero (the steady-state common case).  Short tasks paid full
mutex overhead per completion.  Add a relaxed _remove_count.load
before the locks; if it reads zero, skip the claim entirely.  A stale
zero just defers the claim to a later completion on any worker,
matching the existing design where the claim is a cooperative race.

vendored/libfuse/include/thread_pool.hpp

@@ -546,7 +546,13 @@ ThreadPool::_process_func(Func &func_)
   // prevent the TOCTOU race with set_threads.  During shutdown
   // (_stop set), _try_claim_and_remove_self returns false, so
   // the destructor still joins us.
-  if(_try_claim_and_remove_self(pthread_self()))
+  //
+  // Fast path: most tasks complete with no shrink pending, so avoid
+  // the double-lock acquisition in _try_claim_and_remove_self unless
+  // _remove_count looks non-zero.  A stale zero read just defers the
+  // claim by one task — a later completion on any worker picks it up.
+  if(_remove_count.load(std::memory_order_relaxed) != 0 &&
+     _try_claim_and_remove_self(pthread_self()))
     return EXIT;
 
   return CONTINUE;