selftests: forwarding: multiple instances in tcpdump helper

Extend tcpdump_start() & C:o to handle multiple instances.  Useful when
observing bridge operation, e.g., unicast learning/flooding, and any
case of multicast distribution (to these ports but not that one ...).

This means the interface argument is now a mandatory argument to all
tcpdump_*() functions, hence the changes to the ocelot flower test.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>