Return 0 from gsm0408_rcvmsg() if SCCP link is already closed.

Whether to forward the message or not to an SCCP connection is
an internal question for the GSM 04.08 code. Unlike errors with
the message decoding, memory allocation and other critical errors,
this not an error which should be reported to the caller.
abis_rsl_rx_rll() (the caller) shouldn't know about the message
routing decisions and should only care about actual errors.

This code path is hit in production very often because we frequently
receive a Classmark Change message from a phone right after the MSC
has shut the SCCP connection but before we close the lchan on the BTS.

Change-Id: I2d430ebc894a2345bebaa1841a75e94a3b45eae2
1 file changed