stream: Return 0 when receiving sctp notification SCTP_COMM_LOST

It was seen on a real pcap trace (sctp & gsmtap_log) that the Linux
kernel stack may decide to kill the connection (sending an ABORT) if
it fails to transmit some data after a while:
ABORT Cause code: "Protocol violation (0x000d)",
      Cause Information: "Association exceeded its max_retrans count".
When this occurs, the kernel sends the
MSG_NOTIFICATION,SCTP_ASSOC_CHANGE,SCTP_COMM_LOST notification when
reading from the socket with sctp_recvmsg(). This basically signals that
the socket conn is death, and subsequent writes to it will result in
send() failures (and receive SCTP_SEND_FAILED notification upon follow
up reads).
It's important to notice that after those events, there's no other sort
of different event like SHUTDOWN coming in, so that's the time at which
we must tell the user to close the socket.
Hence, let's signal the caller that the socket is dead by returning 0,
to comply with usual recv() API.

Related: SYS#6113
Change-Id: If94d44f25b76a96a5ea402fec9fc14c4e6296ba3
---
M src/stream.c
1 file changed, 2 insertions(+), 1 deletion(-)