ipv6: Copy cork options in ip6_append_data

As the options passed to ip6_append_data may be ephemeral, we need
to duplicate it for corking.  This patch applies the simplest fix
which is to memdup all the relevant bits.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>