[NET]: Add netdev_alloc_skb().

Add a dev_alloc_skb variant that takes a struct net_device * paramater.
For now that paramater is unused, but I'll use it to allocate the skb
from node-local memory in a follow-up patch.  Also there have been some
other plans mentioned on the list that can use it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>