dsa: slave: add support for TC_SETUP_FT

The dsa infrastructure provides a well-defined hierarchy of devices,
pass up the call to set up the flow block to the master device. From the
software dataplane, the netfilter infrastructure uses the dsa slave
devices to refer to the input and output device for the given skbuff.
Similarly, the flowtable definition in the ruleset refers to the dsa
slave port devices.

This patch adds the glue code to call ndo_setup_tc with TC_SETUP_FT
with the master device via the dsa slave devices.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>