mlxsw: Create dedicated field for Rx metadata in skb control block

Next patch will need to encode more Rx metadata in the skb control
block, so create a dedicated field for it and move the cookie index
there.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>