net offloading: Convert dev_gso_segment() to use precomputed features.

This switches dev_gso_segment() to use the device features computed
by the centralized routine.  In doing so, it fixes a problem where
it would always use dev->features, instead of those appropriate
to the number of vlan tags if any are present.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>