selftests: devlink: Export functions to devlink library

l2_drops_test() is used to check that drop traps are functioning as
intended. Currently it is only used in the layer 2 test, but it is also
useful for the layer 3 test introduced in the subsequent patch.

l2_drops_cleanup() is used to clean configurations and kill mausezahn
proccess.

Export the functions to the common devlink library to allow it to be
re-used by future tests.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>