MDEV-26827 fixup: Fix os_aio_wait_until_no_pending_writes()

io_callback(): Process the request before releasing the write slot.
Before commit a091d6a
when we had a duplicated counter for writes, either ordering was fine.
Now, correctness depends on os_aio_wait_until_no_pending_writes().

storage/innobase/os/os0file.cc

@@ -3457,9 +3457,8 @@ static void io_callback(tpool::aiocb *cb)
   else
   {
     ut_ad(write_slots->contains(cb));
-    const IORequest req{request};
+    fil_aio_callback(request);
     write_slots->release(cb);
-    fil_aio_callback(req);
   }
 }