net: use skb_unref() in napi_consume_skb()

The commit 83ada39bb79d ("net: factor out a helper to decrement the
skb refcount") provided and used a helper for decrementing skb usage,
but I missed at least a spot for it.

This change remove some more duplicated code reusing skb_unref() in
napi_consume_skb(), too. The helper uses an additional, unneeded
unlikely(!skb) test - napi_consume_skb() already check it a few lines
above - but the compiler is smart enough to optimize the duplicated
test out.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>