s390/qeth: allocate skb from NAPI cache

napi_alloc_skb() doesn't need to disable IRQs during the allocation,
and thus may save us a few cycles.
Doing so requires a small fix-up in the HiperTransport path, which
currently assumes a fixed NET_SKB_PAD headroom padding. napi_alloc_skb()
adds an additional NET_IP_ALIGN padding, so use the proper helper for
setting up the mac_header offset.

Use this opportunity to convert the non-NAPI path to netdev_alloc_skb(),
which means that skb->dev is now always set-up during allocation and
doesn't need to be assigned manually.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>