igb: use buffer_info->dma instead of shinfo->nr_frags to determine unmap

This change makes it so that we use buffer_info->dma instead of
shinfo->nr_frags to determine if we need to unmap a received skb.  By doing
this we can avoid a cache miss on small packets since the buffer_info
structure should already be prefetched.

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>