bna: use netdev_alloc_skb_ip_align()

Some workloads need some headroom (NET_SKB_PAD) to avoid expensive
reallocations.

Using netdev_alloc_skb_ip_align() instead of bare skb_alloc() brings the
NET_IP_ALIGN and the NET_SKB_PAD headroom.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Rasesh Mody <rmody@brocade.com>
CC: Debashis Dutt <ddutt@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>