datapth: Drop check for impossible condition after skb_gso_segment().

It's possible for skb_gso_segment to return NULL but only if the
hardware supports the correct form of segmentation offload but just
wants software to verify the offload parameters.  However, since we're
not hardware and don't support any kind of segmentation offload natively,
we can never get in this situation.  Therefore drop the check and
comment.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>