net/sched: cls_flower: Allow setting encapsulation fields as used key

When encapsulation field is set, mark it as used key for the flow
dissector. This will be used by offloading drivers.

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>