qeth: enable scatter/gather in layer 2 mode

The patch enables NETIF_F_SG flag for OSA in layer 2 mode.
It also adds performance accounting for fragmented sends,
adds a conditional skb_linearize() attempt if the skb had
too many fragments for QDIO SBAL, and fills netdevice->gso_*
attributes.

Signed-off-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Reviewed-by: Lakhvich Dmitriy <ldmitriy@ru.ibm.com>
Reviewed-by: Thomas Richter <tmricht@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>