igb: Rework how netdev->stats is handled

This patch does some refactoring work that I felt was needed after reviewing
the changes recently submitted relating to the replacement of net_stats with
netdev->stats.

This patch essentially creates two different collections of stats.  The
first handles the adapter specific states and is stored in gstring_stats,
and the second is for netdev specific stats and is stored in
gstring_net_stats.

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>