FreeBSD: do not panic on dma_buf_poll

Some programs (waypipe) use poll to check inherited file descriptors,
and some other programs (terminal emulators?) can have non-CLOEXEC dmabufs
that leak into waypipe, causing a panic.

drivers/dma-buf/dma-buf.c

@@ -197,8 +197,8 @@ static int
 dma_buf_poll(struct file *fp, int events,
 	     struct ucred *active_cred, struct thread *td)
 {
-	panic("XXX implement me!!!");
-	return (0);
+	// TODO: implement this when we need it (mostly used for cross-device fence sync)
+	return (ENOTSUP);
 }