testing: selftests: net: forwarding: add devlink-required functionality to...

testing: selftests: net: forwarding: add devlink-required functionality to test (hard) dropped stats field

Add devlink_trap_drop_packets_get function, as well as test that are
used to verify devlink (hard) dropped stats functionality works.

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