net/sched: act_tunnel_key: add helper inlines to access tcf_tunnel_key

Needed for drivers to pick the relevant action when offloading tunnel
key act.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>