Fixing body fd access racing condition.

To avoid closing the body fd prematurely, the fd value is moved from
the request struct to the app link.  The body fd should not be closed
immediately after the request is sent to the application due to possible
request rescheduling.

src/nxt_router_request.h

@@ -50,6 +50,7 @@ struct nxt_request_app_link_s {
     nxt_http_request_t      *request;
     nxt_msg_info_t          msg_info;
     nxt_request_rpc_data_t  *req_rpc_data;
+    nxt_fd_t                body_fd;
 
     nxt_nsec_t              res_time;