osmo_ortp: Fix seqno reset to 0 upon ssrc_changed
osmo_ortp: Fix seqno reset to 0 upon ssrc_changed

It was spotted in an osmo-mgw instance receiving RTP from osmo-bts 1.2.3
that sometimes the seqno was reset to 0 (while tx_timestamp was kept
properly). This seems to be the only possible way osmo-bts can reset the
seqno to 0 (calling rtp_session_reset()), since osmo_rtp_socket_create
already sets it to other values and inside ortp rtp_session_reset() is
the only one setting it to 0.
If SSRC changes, we are fine resetting some Rx state, which is done by
calling rtp_session_resync. However, calling rtp_session_reset() is too
much, since it erases other stuff like Tx state, which sholdn't be
affected in this case, since we are still transmitting the same stream.

Related: OS#3104
Related: OS#3299
Related: SYS#5498
Fixes: 28eeb6bc9365bbe47125775fcb1f696317c22466

Change-Id: Ic6006b4991c85413b009b70f8f3a6f97c41990ec

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis