9pfs: xattrcreate requires non-opened fids

The xattrcreate operation only makes sense on a freshly cloned fid
actually, since any open state would be leaked because of the fid_type
change. This is indeed what the linux kernel client does:

	fid = clone_fid(fid);
	[...]
	retval = p9_client_xattrcreate(fid, name, value_len, flags);

This patch also reverts commit ff55e94d23ae since we are sure that a fid
with type P9_FID_NONE doesn't have a previously allocated xattr.

Signed-off-by: Greg Kurz <groug@kaod.org>
