net: Always poll at least one device in net_rx_action

We should only perform the softnet_break check after we have polled
at least one device in net_rx_action.  Otherwise a zero or negative
setting of netdev_budget can lock up the whole system.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>