Currently, the tcp_probe snooper can either filter packets by a given
port (handed to the module via module parameter e.g. port=80) or lets
all TCP traffic pass (port=0, default). When a port is specified, the
port number is tested against the sk's source/destination port. Thus,
if one of them matches, the information will be further processed for
the log.

As this is quite limited, allow for more advanced filtering possibilities
which can facilitate debugging/analysis with the help of the tcp_probe
snooper. Therefore, similarly as added to BPF machine in commit 7e75f93e
("pkt_sched: ingress socket filter by mark"), add the possibility to
use skb->mark as a filter.

If the mark is not being used otherwise, this allows ingress filtering
by flow (e.g. in order to track updates from only a single flow, or a
subset of all flows for a given port) and other things such as dynamic
logging and reconfiguration without removing/re-inserting the tcp_probe
module, etc. Simple example:

  insmod net/ipv4/tcp_probe.ko fwmark=8888 full=1
  ...
  iptables -A INPUT -i eth4 -t mangle -p tcp --dport 22 \
           --sport 60952 -j MARK --set-mark 8888
  [... sampling interval ...]
  iptables -D INPUT -i eth4 -t mangle -p tcp --dport 22 \
           --sport 60952 -j MARK --set-mark 8888

The current option to filter by a given port is still being preserved. A
similar approach could be done for the sctp_probe module as a follow-up.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>