layer23: Terminate process if L2 socket fails/dissappears

We don't recover from this situation at all, so it's best to
terminate the process.

Change-Id: I487d18e4afe2cae9f777a30864e680b5dc821fc1

src/host/layer23/src/common/l1l2_interface.c

@@ -62,6 +62,7 @@ static int layer2_read(struct osmo_fd *fd)
 		if (rc >= 0)
 			rc = -EIO;
 		layer2_close((struct osmocom_ms *) fd->data);
+		exit(102);
 		return rc;
 	}