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: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>