datapath: add skb_clone NULL check in the recirc action.

Refactoring recirc action implementation.

The main change is to fix a bug where the NULL check after skb clone()
call is missing.  The fix is to return -ENOMEM whenever skb_clone()
failed to create a clone.

Also rearranged adjacent code to improve readability.

Reported-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>