hnbgw: Fix recent regression not closing conn upon rx of SCTP_SHUTDOWN_EVENT

Before handling of OSMO_STREAM_SCTP_MSG_FLAGS_NOTIFICATION in recent
commit (see Fixes: below), osmo_stream_srv_recv() and
internal _sctp_recvmsg_wrapper() in libosmo-netif would return either
-EAGAIN or 0 when an sctp notification was received from the kernel.
After adding handling of OSMO_STREAM_SCTP_MSG_FLAGS_NOTIFICATION, the
code paths for "rc == -EAGAIN" and "rc == 0" would not be executed
anymore since the first branch takes preference in the if-else
tree. For "rc == -EAGAIN" it's fine because the new branch superseeds
what's done on the "rc == -EAGAIN" branch. However, for the "rc == 0",
we forgot to actually destroy the connection. The "rc == 0" branch was
basically reached when SCTP_SHUTDOWN_EVENT was received because
osmo_stream_srv_recv() tried to resemble the interface of regular
recv(); let's hence check for that explicitly and destroy the conn object
(and the related hnb context in the process) when we receive that event.

Fixes: 1de2091515530e531196ebca27b1445cecd1b88d
Related: SYS#6113
Change-Id: I11b6af51a58dc250975a696b98d0c0c9ff3df9e0
---
M src/osmo-hnbgw/hnbgw.c
1 file changed, 11 insertions(+), 4 deletions(-)