openvswitch: Fix L4 checksum handling when dealing with IP fragments

openvswitch modifies the L4 checksum of a packet when modifying
the ip address. When an IP packet is fragmented only the first
fragment contains an L4 header and checksum. Prior to this change
openvswitch would modify all fragments, modifying application data
in non-first fragments, causing checksum failures in the
reassembled packet.

Signed-off-by: Glenn Griffin <ggriffin.kernel@gmail.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>