net/act_skbedit: Utility functions for mark action

Enable device drivers to query the action, if and only if is a mark
action and what value to use for marking.

Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Amir Vadai <amir@vadai.me>
Signed-off-by: David S. Miller <davem@davemloft.net>