esp6: Fix scatterlist initialization

When we use IPsec extended sequence numbers, we may overwrite
the last scatterlist of the associated data by the scatterlist
for the skb. This patch fixes this by placing the scatterlist
for the skb right behind the last scatterlist of the associated
data. esp4 does it already like that.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>