netpoll: add IPv6 support

Currently, netpoll only supports IPv4. This patch adds IPv6
support to netpoll so that we can run netconsole over IPv6 network.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>