iptables: set-dscpmark follow upstreamimg attempt

I'm having another attempt at trying to getting the 'store dscp into
conntrack connmark' functionality into upstream kernel, since the
restore function (act_ctinfo) has been accepted.

The syntax has changed from 'savedscp' to 'set-dscpmark' since that
conforms more closely with existing functionality.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>