ixgbe: use rx_buffer_info->dma instead of nr_frags to determine skb unmap

This patch changes the driver so that it uses rx_buffer_info->dma to
determine if it needs to unmap the page instead of sh_info->nr_frags.  This
helps to prevent a cache line miss when receiving small packets as the
rx_buffer_info data should already be in the cache.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>