Fix non-reentrant threadpool (#5714)

Summary:
Pull Request resolved: #5714

If we tried to reenter parallel_for, we would deadlock due to trying to acquire the threadpool mutex, which was already held. Removing the mutex (which is unnecessary) didn't fix the problem because pthreadpool has its own internal mutex.

Reviewed By: kimishpatel

Differential Revision: D62052200

fbshipit-source-id: 1be16d581970fa7b6a84a2f5b4e348b4f5069c80

Author: swolchok

extension/threadpool/threadpool.cpp

@@ -86,6 +86,7 @@ void ThreadPool::run(
       // pthreadpool_parallelize_1d() cannot go out of scope until
       // pthreadpool_parallelize_1d() returns.
       [](void* const context, const size_t item) {
+        NoThreadPoolGuard guard;
         reinterpret_cast<Context*>(context)->fn(item);
       },
       &context,