selftests: mptcp: add packet mark test case

Extend mptcp_connect tool with SO_MARK support (-M <value>) and
add a test case that checks that the packet mark gets copied to all
subflows.

This is done by only allowing packets with either skb->mark 1 or 2
via iptables.

DROP rule packet counter is checked; if its not zero, print an error
message and fail the test case.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>