xfrm: interface: do not confirm neighbor when do pmtu update

When do IPv6 tunnel PMTU update and calls __ip6_rt_update_pmtu() in the end,
we should not call dst_confirm_neigh() as there is no two-way communication.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>