net: dsa: ksz: fix skb freeing

The DSA layer frees the original skb when an xmit function returns NULL,
meaning an error occurred. But if the tagging code copied the original
skb, it is responsible of freeing the copy if an error occurs.

The ksz tagging code currently has two issues: if skb_put_padto fails,
the skb copy is not freed, and the original skb will be freed twice.

To fix that, move skb_put_padto inside both branches of the skb_tailroom
condition, before freeing the original skb, and free the copy on error.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>