Changes

lb: Avoid using osmo_ss7_instance private field

We know the id because we just found/created it base don that id, so
simply use it instead of accessing the library object field.

Change-Id: Ib6d1f4898eb0a804ebda527fdd876d2ca4ec1e67
(cherry picked from commit d812fb1e98001f99e2f814ea52292ebdfe03a52f)