igb: use netif_carrier_off earlier when bringing if down

Use netif_carrier_off() first, since that will prevent the stack from
queuing more packets to this IF. This operation is fast, and should
behave much nicer when trying to bring down an interface under load.

Reported-by: Eliezer Tamir <eliezer.tamir@linux.intel.com>
Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>