IP_GRE: Fix GRE_CSUM case.

commit "ip_gre: allow CSUM capable devices to handle packets"
aa0e51cdda005cd37e2, broke GRE_CSUM case.
GRE_CSUM needs checksum computed for inner packet. Therefore
csum-calculation can not be offloaded if tunnel device requires
GRE_CSUM.  Following patch fixes it by computing inner packet checksum
for GRE_CSUM type, for all other type of GRE devices csum is offloaded.

CC: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>