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>

net/ipv6/ip6_icmp.c

@@ -0,0 +1,47 @@
+#include <linux/export.h>
+#include <linux/icmpv6.h>
+#include <linux/mutex.h>
+#include <linux/netdevice.h>
+#include <linux/spinlock.h>
+
+#include <net/ipv6.h>
+
+#if IS_ENABLED(CONFIG_IPV6)
+
+static ip6_icmp_send_t __rcu *ip6_icmp_send;
+
+int inet6_register_icmp_sender(ip6_icmp_send_t *fn)
+{
+	return (cmpxchg((ip6_icmp_send_t **)&ip6_icmp_send, NULL, fn) == NULL) ?
+	        0 : -EBUSY;
+}
+EXPORT_SYMBOL(inet6_register_icmp_sender);
+
+int inet6_unregister_icmp_sender(ip6_icmp_send_t *fn)
+{
+	int ret;
+
+	ret = (cmpxchg((ip6_icmp_send_t **)&ip6_icmp_send, fn, NULL) == fn) ?
+	      0 : -EINVAL;
+
+	synchronize_net();
+
+	return ret;
+}
+EXPORT_SYMBOL(inet6_unregister_icmp_sender);
+
+void icmpv6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info)
+{
+	ip6_icmp_send_t *send;
+
+	rcu_read_lock();
+	send = rcu_dereference(ip6_icmp_send);
+
+	if (!send)
+		goto out;
+	send(skb, type, code, info);
+out:
+	rcu_read_unlock();
+}
+EXPORT_SYMBOL(icmpv6_send);
+#endif