sunvnet: don't change gso data on clones

This patch unclones an skb for the case where the sunvnet driver needs to
change the segmentation size so that it doesn't interfere with TCP SACK's
use of them.

Signed-off-by: David L Stevens <david.stevens@oracle.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>