selftests: pass pref and handle to devlink_trap_drop_* helpers

Currently the helpers assume pref 1 and handle 101. Make that explicit
and pass the values from callers.

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