netpoll: Fix missing TXQ unlock and and OOPS.

[ Upstream commit aca5f58f9ba803ec8c2e6bcf890db17589e8dfcc ]

The VLAN tag handling code in netpoll_send_skb_on_dev() has two problems.

1) It exits without unlocking the TXQ.

2) It then tries to queue a NULL skb to npinfo->txq.

Reported-by: Ahmed Tamrawi <atamrawi@iastate.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>