totempg: totempg_groups_join return valid error

totempg_groups_join() is called by sync_init().
sync_init() judge that totempg_groups_join() failed if return code of
totempg_groups_join() is -1.
Therefore, the return code should return in -1 when
totempg_groups_join() fails.

Signed-off-by: Takeshi MIZUTA <miz.take4@gmail.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>