MDEV-25953 Tpool - prevent potential deadlock in simulated AIO

Do not execute user callback just after pwrite. Instead, submit user
function as task into thread pool. This way, the IO thread would not hog
aiocb, which is a limited (in Innodb) resource

tpool/aio_simulated.cc

@@ -159,14 +159,17 @@ class simulated_aio : public aio
 #endif
     cb->m_ret_len = ret_len;
     cb->m_err = err;
-    cb->m_callback(cb);
+    cb->m_internal_task.m_func= cb->m_callback;
+    thread_pool *pool= (thread_pool *)cb->m_internal;
+    pool->submit_task(&cb->m_internal_task);
   }
 
   virtual int submit_io(aiocb *aiocb) override
   {
     aiocb->m_internal_task.m_func = simulated_aio_callback;
     aiocb->m_internal_task.m_arg = aiocb;
     aiocb->m_internal_task.m_group = aiocb->m_group;
+    aiocb->m_internal = m_pool;
     m_pool->submit_task(&aiocb->m_internal_task);
     return 0;
   }