drivers: net: netdevsim: add devlink trap_drop_counter_get implementation

Whenever query statistics is issued for trap with DROP action,
devlink subsystem would also fill-in statistics 'dropped' field.
In case if device driver did't register callback for hard drop
statistics querying, 'dropped' field will be omitted and not filled.
Add trap_drop_counter_get callback implementation to the netdevsim.
Add new test cases for netdevsim, to test both the callback
functionality, as well as drop statistics alteration check.

Signed-off-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>