Due to a bug in np_create_rerror, space is allocated
for the errno in Rerror packets, even when 9P2000.u was
not negotiated, causing protocol errors.
Attached patch changes np_create_rerror to be similar
to np_create_error1 and only allocate the extra 4 bytes
when 9P2000 is negotiated.
patch to only send back 9P2000.u errors when 9P2000.u was successfully negotiated