The old approach was: rtp_data_net() reads a msgb from the incomging
socket, calls through whatever function chain and in the end free's it.
So none of the intermediate functions was permitted to take msgb
ownership.
This was a good choice as all processing would happen synchronously,
up to the point where that msgb was written on the output RTP socket.
Let's change this from passing msgb ownership throug the whole call
chain, through rx_rtp() to the various *_dispatch_rtp() functions.
This is required for upcoming migration to osmo_io, as in that case the
write (sendto) calls are asynchronous and hence msgb ownership needs
to be transferred.
Change-Id: I6a331f3c6b2eb51ea312ac6ef8c357185ddb79cf