vxlan: fix receive checksum handling

Vxlan was trying to use postpull_rcsum to allow receive checksum
offload to work on drivers using CHECKSUM_COMPLETE method. But this
doesn't work correctly. Just force full receive checksum on received
packet.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>