GPRS: We have to do the msgb_free() in NS not Gb Proxy

As only NS-UNITDATA messages are ever passed into the Gb Proxy,
we need to do the msgb_free() at a much higher point in the calling
stack, i.e. inside the NS protocol layer.  This means it is now
the same logic as in OpenBSC itself.
diff --git a/openbsc/src/gprs/gprs_ns.c b/openbsc/src/gprs/gprs_ns.c
index 1b9d7c6..11637f7 100644
--- a/openbsc/src/gprs/gprs_ns.c
+++ b/openbsc/src/gprs/gprs_ns.c
@@ -793,7 +793,11 @@
 	if (!msg)
 		return error;
 
-	return gprs_ns_rcvmsg(nsi, msg, &saddr);
+	error = gprs_ns_rcvmsg(nsi, msg, &saddr);
+
+	msgb_free(msg);
+
+	return error;
 }
 
 static int handle_nsip_write(struct bsc_fd *bfd)