ibmveth: Convert to netdev_alloc_skb

We were using alloc_skb which doesn't create any headroom. Change it to
use netdev_alloc_skb to match most other drivers.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Santiago Leon <santil@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>