l1sap: Validate incoming RTP payload, drop bw-efficient AMR
l1sap: Validate incoming RTP payload, drop bw-efficient AMR

A recurrent kernel crash in sysmobts (several kernel versions)
corrupting kernel memory in random places has been investigated and
reproduced by placing a call against an MSC sending RTP
with bandwidth-efficient AMR payload to osmo-bts-sysmo.
The osmo-bts-sysmo in turn sends the payload to the femtobts related
kernel modules via a msgq, which most probably fail to handle correctly
this bw-efficient AMR payload and corrupt the kernel memory.

First approach was to drop the bw-efficient AMR payloads lower in the
stack in sysmo specific code (l1if_tch_encode), but as there's no bts
model in osmo-bts actually supporting bw-efficient AMR, let's drop it
early in the incoming path for all models to avoid further problems.

Related: SYS#4063

Change-Id: If0c9233c628c724de4ab74e58e3e2affac79e6d0

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts