net/sched: act_csum: Add accessors for offloading drivers

Add the accessors for realizing if this is a csum action,
and for which fields checksum is needed.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>