[NET] netpoll: don't spin forever sending to stopped queues

When transmitting a skb in netpoll_send_skb(), only retry a limited number
of times if the device queue is stopped.

Signed-off-by: Jeremy Fitzhardinge <jeremy@goop.org>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>