ip_gre: Fix WCCPv2 header parsing.

[ No applicable upstream commit, the upstream implementation is
  now completely different and doesn't have this bug. ]

In case of WCCPv2 GRE header has extra four bytes.  Following
patch pull those extra four bytes so that skb offsets are set
correctly.

CC: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Peter Schmitt <peter.schmitt82@yahoo.de>
Tested-by: Peter Schmitt <peter.schmitt82@yahoo.de>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>