vmxnet3: use netdev_alloc_skb_ip_align

Use netdev_alloc_skb_align, rather than open code using dev_alloc_skb.
Change allocation at startup to use GFP_KERNEL.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>