kernel: netfilter add connmark savedscp support

savedscp is a method of storing the DSCP of an ip packet into conntrack
mark.  In combination with a suitable tc filter action (conndscp but may
end up being integrated into connmark) DSCP values are able to be stored
on egress and restored on ingress across links that otherwise alter or
bleach DSCP.

This is useful for qdiscs such as CAKE which are able to shape according
to policies based on DSCP.

Ingress classification is traditionally a challenging task since
iptables rules haven't yet run and tc filter/eBPF programs are pre-NAT
lookups, hence are unable to see internal IPv4 addresses as used on the
typical home masquerading gateway.

The ingress problem is solved by the tc filter, but the tc people didn't
like the idea of tc setting conntrack mark values, though they are ok
with reading conntrack values and hence restoring DSCP from conntrack
marks.

x_tables CONNMARK with the new savedscp action solves the problem of
storing the DSCP to the conntrack mark.

It accepts 2 parameters.  The mark is a 32bit value with usually one 1
bit set.  This bit is set when savedscp saves the DSCP to the mark.
This is useful to implement a 'one shot'
iptables based classification where the 'complicated' iptables rules are
only run once to classify the connection on initial (egress) packet and
subsequent packets are all marked/restored with the same DSCP.  A mark
of zero disables the setting of a status bit/s.

The mask is a 32bit value of at least 6 contiguous bits and represents
the area where the DSCP will be stored.

e.g.

iptables -A QOS_MARK_eth0 -t mangle -j CONNMARK --savedscp-mark 0xfc000000/0x01000000

Would store the DSCP in the top 6 bits of the 32bit mark field, and use
the LSB of the top byte as the 'DSCP has been stored' marker.

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

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