netpoll: fix the skb check in pkt_is_ns

[ Not applicable upstream commit, the code here has been removed
  upstream. ]

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>