ipv6: Kill ipv6 dependency of icmpv6_send().

Following patch adds icmp-registration module for ipv6.  It allows
ipv6 protocol to register icmp_sender which is used for sending
ipv6 icmp msgs.  This extra layer allows us to kill ipv6 dependency
for sending icmp packets.

This patch also fixes ip_tunnel compilation problem when ip_tunnel
is statically compiled in kernel but ipv6 is module

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>