openvswitch: actions: use skb_postpull_rcsum when possible

Replace duplicated code by calling skb_postpull_rcsum

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>