tun: Use POLLERR not EBADF in tun_chr_poll

EBADF is meaningless in the context of a poll mask so use POLLERR
instead.
Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -382,7 +382,7 @@ static unsigned int tun_chr_poll(struct file *file, poll_table * wait)
 	unsigned int mask = POLLOUT | POLLWRNORM;
 
 	if (!tun)
-		return -EBADFD;
+		return POLLERR;
 
 	DBG(KERN_INFO "%s: tun_chr_poll\n", tun->dev->name);