netfilter: ip6t_SYNPROXY: remove magic number for hop_limit

Replace '64' with the per-net ipv6_devconf_all's hop_limit when
building the ipv6 header.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>