net/vxlan: Share RX skb de-marking and checksum checks with ovs

Make sure the practice set by commit 0afb1666 "vxlan: Add capability
of Rx checksum offload for inner packet" is applied when the skb
goes through the portion of the RX code which is shared between
vxlan netdevices and ovs vxlan port instances.

Cc: Joseph Gasparakis <joseph.gasparakis@intel.com>
Cc: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>