Polish insignificant data race

Coverity CID#469225

src/fellow_cache.c

@@ -3013,12 +3013,12 @@ static void *
 fellow_cache_async_thread(void *priv)
 {
 	struct fellow_cache *fc;
-	unsigned n;
+	unsigned n, entries;
 
 	CAST_OBJ_NOTNULL(fc, priv, FELLOW_CACHE_MAGIC);
-	unsigned entries = fellow_io_entries(fc->async_ioctx);
 
 	AZ(pthread_mutex_lock(&fc->async_mtx));
+	entries = fellow_io_entries(fc->async_ioctx);
 	while (fc->running) {
 		struct fellow_io_status status[entries];
 		n = fellow_io_wait_completions(fc->async_ioctx,