Don't return `channel_state` from `decode_update_add_htlc_onion`

Don't return `channel_state` from `decode_update_add_htlc_onion`

Currently `decode_update_add_htlc_onion` returns the `channel_state`
lock to ensure that `internal_update_add_htlc` holds a single
`channel_state` lock in when the entire function execution. This is
unnecessary, and since we are moving the channel storage to the
`per_peer_state`, this no longer achieves the goal it was intended for.

We therefore avoid returning the `channel_state` from
`decode_update_add_htlc_onion`, and just retake the lock in
`internal_update_add_htlc` instead.