selftests: devlink: Make devlink_trap_cleanup() more generic

Add proto parameter in order to enable the use of devlink_trap_cleanup()
in tests that use IPv6 protocol.

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>