ip6_gre: do not confirm neighbor when do pmtu update

ip6_gre: do not confirm neighbor when do pmtu update

When we do ipv6 gre pmtu update, we will also do neigh confirm currently.
This will cause the neigh cache be refreshed and set to REACHABLE before
xmit.

But if the remote mac address changed, e.g. device is deleted and recreated,
we will not able to notice this and still use the old mac address as the neigh
cache is REACHABLE.

Fix this by disable neigh confirm when do pmtu update

v5: No change.
v4: No change.
v3: Do not remove dst_confirm_neigh, but add a new bool parameter in
    dst_ops.update_pmtu to control whether we should do neighbor confirm.
    Also split the big patch to small ones for each area.
v2: Remove dst_confirm_neigh in __ip6_rt_update_pmtu.

Reported-by: Jianlin Shi <jishi@redhat.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>