read_callback: move to SessionHandle from connectdata

With many easy handles using the same connection for multiplexing, it is
important we store and keep the transfer-oriented stuff in the
SessionHandle so that callbacks and callback data work fine even when
many easy handles share the same physical connection.

lib/ssh.c

@@ -1737,8 +1737,8 @@ static CURLcode ssh_statemach_act(struct connectdata *conn, bool *block)
                 BUFSIZE : curlx_sotouz(data->state.resume_from - passed);
 
               size_t actuallyread =
-                conn->fread_func(data->state.buffer, 1, readthisamountnow,
-                                 conn->fread_in);
+                data->set.fread_func(data->state.buffer, 1, readthisamountnow,
+                                     data->set.in);
 
               passed += actuallyread;
               if((actuallyread == 0) || (actuallyread > readthisamountnow)) {