mount.ceph: behave when CONFIG_KEYS is not compiled in
mount.ceph: behave when CONFIG_KEYS is not compiled in

In that case we get ENOSYS.  This also implies an old version of the client
and that we should fall back.

Signed-off-by: Sage Weil <c5b6ff1fd4f0bde10db63d7a3e07166cf6c4229f@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph