libceph: rename ceph_entity_name_encode() -> ceph_auth_entity_name_encode()

Clear up EntityName vs entity_name_t confusion.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Alex Elder <elder@linaro.org>