netdevsim: add warnings on unexpected UDP tunnel port errors

We should never see a removal of a port which is not in the table
or adding a port to an occupied entry in the table. To make sure
such errors don't escape the checks in the test script add a
warning/kernel spat.

Error injection will not trigger those, nor should it ever put
us in a bad state.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>