client: endp fsm: add osmo_mgcpc_ep_ci_ep()

If an API user only has access to the ci FSM (which is managed via an opaque
struct), provide this function to obtain the backpointer to the parent endpoint
FSM, mostly to be able to call osmo_mgcpc_ep_cancel_notify() on it.

osmo-msc's rtp_stream FSM will use this in
I351bb8e8fbc46eb629bcd599f6453e2c84c15015.

Change-Id: I14f7a46031327fb2b2047b998eae6ad0bb7324ad
2 files changed