igb: update stats before doing reset in igb_down

It was seen with repeated interface up/down testing that there was a large
stray between the stats reported by the queues and the stats reported by the
HW.  It was found to be an issue in that hw stats were being reset without
first being recorded.  This change records the stats before wiping them from
the system via the reset.

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>