Add flag 'FWMARK' to enable use of firewall connmarks as tin selector.
The connmark (skbuff->mark) needs to be in the range 1->tin_cnt ie.
for diffserv3 the mark needs to be 1->3.

Background

Typically CAKE uses DSCP as the basis for tin selection.  DSCP values
are relatively easily changed as part of the egress path, usually with
iptables & the mangle table, ingress is more challenging.  CAKE is often
used on the WAN interface of a residential gateway where passthrough of
DSCP from the ISP is either missing or set to unhelpful values thus use
of ingress DSCP values for tin selection isn't helpful in that
environment.

An approach to solving the ingress tin selection problem is to use
CAKE's understanding of tc filters.  Naive tc filters could match on
source/destination port numbers and force tin selection that way, but
multiple filters don't scale particularly well as each filter must be
traversed whether it matches or not. e.g. a simple example to map 3
firewall marks to tins:

MAJOR=$( tc qdisc show dev $DEV | head -1 | awk '{print $3}' )
tc filter add dev $DEV parent $MAJOR protocol all handle 0x01 fw action skbedit priority ${MAJOR}1
tc filter add dev $DEV parent $MAJOR protocol all handle 0x02 fw action skbedit priority ${MAJOR}2
tc filter add dev $DEV parent $MAJOR protocol all handle 0x03 fw action skbedit priority ${MAJOR}3

Another option is to use eBPF cls_act with tc filters e.g.

MAJOR=$( tc qdisc show dev $DEV | head -1 | awk '{print $3}' )
tc filter add dev $DEV parent $MAJOR bpf da obj my-bpf-fwmark-to-class.o

This has the disadvantages of a) needing someone to write & maintain
the bpf program, b) a bpf toolchain to compile it and c) needing to
hardcode the major number in the bpf program so it matches the cake
instance (or forcing the cake instance to a particular major number)
since the major number cannot be passed to the bpf program via tc
command line.

As already hinted at by the previous examples, it would be helpful
to associate tins with something that survives the Internet path and
ideally allows tin selection on both egress and ingress.  Netfilter's
conntrack permits setting an identifying mark on a connection which
can also be restored to an ingress packet with tc action connmark e.g.

tc filter add dev eth0 parent ffff: protocol all prio 10 u32 \
	match u32 0 0 flowid 1:1 action connmark action mirred egress redirect dev ifb1

Since tc's connmark action has restored any connmark into skb->mark,
any of the previous solutions are based upon it and in one form or
another copy that mark to the skb->priority field where again CAKE
picks this up.

This change cuts out at least one of the (less intuitive &
non-scalable) middlemen and permit direct access to skb->mark.
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>