net: use netif_rx_ni() from process context

Hotpluging a cpu might be rare, yet we have to use proper
handlers when taking over packets found in backlog queues.

dev_cpu_callback() runs from process context, thus we should
call netif_rx_ni() to properly invoke softirq handler.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>