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>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 20ef14d..8743de9 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -382,7 +382,7 @@
 	unsigned int mask = POLLOUT | POLLWRNORM;
 
 	if (!tun)
-		return -EBADFD;
+		return POLLERR;
 
 	DBG(KERN_INFO "%s: tun_chr_poll\n", tun->dev->name);