s390: scatter-gather for inbound traffic in qeth driver

For large incoming packets > PAGE_SIZE/2 qeth creates a fragmented skb
by adding pointers to qdio pages to the fragment list of the skb.
This avoids allocating big chunks of consecutive memory. Also copying
data from the qdio buffer to the skb is economized.

Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Frank Pavlic <fpavlic@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>