igb: Unpair the queues when changing the number of queues

BugLink: http://bugs.launchpad.net/bugs/1547674

By the commit 72ddef05 ("igb: Fix oops caused by missing queue
pairing"), the IGB_FLAG_QUEUE_PAIRS flag can now be set when changing the
number of queues by "ethtool -L", but it is never cleared unless the igb
driver is reloaded.
This patch clears it if queue pairing becomes unnecessary as a result of
"ethtool -L".
Signed-off-by: Shota Suzuki <suzuki_shota_t3@lab.ntt.co.jp>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from net-next commit 37a5d163)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -2931,6 +2931,8 @@ void igb_set_flag_queue_pairs(struct igb_adapter *adapter,
 		 */
 		if (adapter->rss_queues > (max_rss_queues / 2))
 			adapter->flags |= IGB_FLAG_QUEUE_PAIRS;
+		else
+			adapter->flags &= ~IGB_FLAG_QUEUE_PAIRS;
 		break;
 	}
 }