libceph: Fix NULL pointer dereference in auth client code

commit 2cb33cac622afde897aa02d3dcd9fbba8bae839e upstream.

A malicious monitor can craft an auth reply message that could cause a
NULL function pointer dereference in the client's kernel.

To prevent this, the auth_none protocol handler needs an empty
ceph_auth_client_ops->build_request() function.

CVE-2013-1059

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Reported-by: Chanam Park <chanam.park@hkpco.kr>
Reviewed-by: Seth Arnold <seth.arnold@canonical.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>