net_sched: act_vlan: add helper inlines to access tcf_vlan info

Needed e.g for offloading drivers to pick the relevant attributes.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>