netfilter: xt_conntrack: Support bit-shifting for CONNMARK & MARK targets.

This patch introduces a new feature that allows bitshifting (left
and right) operations to co-operate with existing iptables options.

Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Jack Ma <jack.ma@alliedtelesis.co.nz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>