ehea: Allocate large enough skbs to avoid partial cacheline DMA writes

The ehea adapter has a mode where it will avoid partial cacheline DMA
writes on receive by always padding packets to fall on a cacheline
boundary.

Unfortunately we currently aren't allocating enough space for a full
ethernet MTU packet to be rounded up, so this optimisation doesn't hit.

It's unfortunate that the next largest packet size exposed by the
hypervisor interface is 2kB, meaning our skb allocation comes out of a
4kB SLAB. However the performance increase due to this optimisation is
quite large and my TCP stream numbers increase from 900MB to 1000MB/sec.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>