openvswitch: Refactor recirc key allocation.

The logic of allocating and copy key for each 'exec_actions_level'
was specific to execute_recirc(). However, future patches will reuse
as well.  Refactor the logic into its own function clone_key().

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>