Refactored sendfile() AIO preload. This reduces layering violation and simplifies the logic of AIO preread, since it's now triggered by the send chain function itself without falling back to the copy filter. The context of AIO operation is now stored per file buffer, which makes it possible to properly handle cases when multiple buffers come from different locations, each with its own configuration.

--- a/src/core/ngx_buf.h
+++ b/src/core/ngx_buf.h
@@ -94,6 +94,9 @@ struct ngx_output_chain_ctx_s {
     unsigned                     aio:1;
 
     ngx_output_chain_aio_pt      aio_handler;
+#if (NGX_HAVE_FILE_AIO)
+    ssize_t                    (*aio_preload)(ngx_buf_t *file);
+#endif
 #endif
 
     off_t                        alignment;