9pfs: check the size of transport buffer before marshaling

v9fs_do_readdir_with_stat() should check for a maximum buffer size
before an attempt to marshal gathered data. Otherwise, buffers assumed
as misconfigured and the transport would be broken.

The patch brings v9fs_do_readdir_with_stat() in conformity with
v9fs_do_readdir() behavior.

Signed-off-by: Jan Dakinevich <jan.dakinevich@gmail.com>
[groug, regression caused my commit 8d37de41 # 2.10]
Signed-off-by: Greg Kurz <groug@kaod.org>