The commit d75b1ade (net: less
interrupt masking in NAPI) required drivers to leave poll_list
empty if the entire budget is consumed.

We have already had two broken drivers so let's add a check for
this.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>