ipv6: fix incorrent ipv6 ipsec packet fragment

Since commit 299b0767(ipv6: Fix IPsec slowpath fragmentation problem)
In func ip6_append_data,after call skb_put(skb, fraglen + dst_exthdrlen)
the skb->len contains dst_exthdrlen,and we don't reduce dst_exthdrlen at last
This will make fraggap>0 in next "while cycle",and cause the size of skb incorrent

Fix this by reserve headroom for dst_exthdrlen.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>