xfrm6: fix a potential use after free in xfrm6_policy.c

pskb_may_pull() maybe change skb->data and make nh and exthdr pointer
oboslete, so recompute the nd and exthdr

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>