ibmvnic: Use netdev_alloc_skb instead of alloc_skb to replenish RX buffers

Take advantage of the additional optimizations in netdev_alloc_skb when
allocating socket buffers to be used for packet reception.

Signed-off-by: Dwip N. Banerjee <dnbanerg@us.ibm.com>
Acked-by: Lijun Pan <ljp@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>