netpoll: fix the skb check in pkt_is_ns

Neighbor Solicitation is ipv6 protocol, so we should check
skb->protocol with ETH_P_IPV6

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Cc: WANG Cong <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>