ipv6: Replace spinlock with seqlock and rcu in ip6_tunnel

This patch uses a seqlock to ensure consistency between idst->dst and
idst->cookie.  It also makes dst freeing from fib tree to undergo a
rcu grace period.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/ip6_tunnel.h
Simple merge

diff --cc net/ipv6/ip6_fib.c
Simple merge

diff --cc net/ipv6/ip6_tunnel.c
Simple merge