tun: Remove unnecessary BUG_ON check in tun_net_xmit

The BUG_ON for NULL tfile is now redundant due to a recently
added null check after the rcu_dereference. Remove the BUG_ON.

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1078,8 +1078,6 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	tun_debug(KERN_INFO, tun, "tun_net_xmit %d\n", skb->len);
 
-	BUG_ON(!tfile);
-
 	/* Drop if the filter does not like it.
 	 * This is a noop if the filter is disabled.
 	 * Filter can be enabled only for the TAP devices. */