openvswitch: update checksum in {push,pop}_mpls

In the case of CHECKSUM_COMPLETE the skb checksum should be updated in
{push,pop}_mpls() as they the type in the ethernet header.

As suggested by Pravin Shelar.

Cc: Pravin Shelar <pshelar@nicira.com>
Fixes: 25cd9ba0

 ("openvswitch: Add basic MPLS support to kernel")
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>