net: dsa: warn if port lists aren't empty in dsa_port_teardown

There has been recent work towards matching each switchdev object
addition with a corresponding deletion.

Therefore, having elements in the fdbs, mdbs, vlans lists at the time of
a shared (DSA, CPU) port's teardown is indicative of a bug somewhere
else, and not something that is to be expected.

We shouldn't try to silently paper over that. Instead, print a warning
and a stack trace.

This change is a prerequisite for moving the initialization/teardown of
these lists. Make it clear that clearing the lists isn't needed.

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