ixgbevf: Fix IPv6 GSO type checks

Based on patch from Sridhar Samudrala

The following patch fixes the check for IPv6 GSO packet in ixgbevf
driver to use skb_is_gso_v6(). SKB_GSO_DODGY is also set
when packets are forwarded from a guest.

CC: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>