libceph: add non-asserting rbtree insertion helper

Needed for the next commit and useful for ceph_pg_pool_info tree as
well.  I'm leaving the asserting helper in for now, but we should look
at getting rid of it in the future.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>