net/sched: Introduce act_tunnel_key

This action could be used before redirecting packets to a shared tunnel
device, or when redirecting packets arriving from a such a device.

The action will release the metadata created by the tunnel device
(decap), or set the metadata with the specified values for encap
operation.

For example, the following flower filter will forward all ICMP packets
destined to 11.11.11.2 through the shared vxlan device 'vxlan0'. Before
redirecting, a metadata for the vxlan tunnel is created using the
tunnel_key action and it's arguments:

$ tc filter add dev net0 protocol ip parent ffff: \
    flower \
      ip_proto 1 \
      dst_ip 11.11.11.2 \
    action tunnel_key set \
      src_ip 11.11.0.1 \
      dst_ip 11.11.0.2 \
      id 11 \
    action mirred egress redirect dev vxlan0

Signed-off-by: Amir Vadai <amir@vadai.me>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>