varnishtest: Remove critical section

The buf_len field is only ever modified by the poll thread and should not
need to be guarded here. This will hopefully silence a coverity warning:

   CID 1485138:  Concurrent data access violations  (ATOMICITY)
   Using an unreliable value of "l" inside the second locked
   section. If the data that "l" depends on was changed by
   another thread, this use might be incorrect.

It might on the other hand complain even harder, I have an alternative
if that happens.

bin/varnishtest/vtc_tunnel.c

@@ -345,9 +345,7 @@ tunnel_write(struct tunnel *t, struct vtclog *vl, struct tunnel_lane *lane,
 	ssize_t res, l;
 
 	p = lane->buf;
-	AZ(pthread_mutex_lock(&t->mtx));
 	l = lane->buf_len;
-	AZ(pthread_mutex_unlock(&t->mtx));
 
 	if (l > 0)
 		vtc_log(vl, 3, "%s %zd bytes", action, l);