datapath: Remove flow member from struct ovs_skb_cb

struct ovs_skb_cb is full on kernels < 3.11 due to compatibility code.
This patch removes the 'flow' member in order to make room for data
needed by layer 3 flow/port support that will be added in an upcoming
patch.  The 'flow' memeber was chosen for removal because it's only used
in ovs_execute_actions().

Signed-off-by: Lorand Jakab <lojakab@cisco.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>