hv_netvsc: Fix accessing freed memory in netvsc_change_mtu()

BugLink: http://bugs.launchpad.net/bugs/1583357

struct netvsc_device is freed in rndis_filter_device_remove(). So we save
the nvdev->num_chn into a temp variable for later usage.

(Please also include this patch into stable branch.)
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit d212b463)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/net/hyperv/netvsc_drv.c

@@ -858,6 +858,7 @@ static int netvsc_change_mtu(struct net_device *ndev, int mtu)
 	struct netvsc_device *nvdev = hv_get_drvdata(hdev);
 	struct netvsc_device_info device_info;
 	int limit = ETH_DATA_LEN;
+	u32 num_chn;
 	int ret = 0;
 
 	if (nvdev == NULL || nvdev->destroy)
@@ -873,6 +874,8 @@ static int netvsc_change_mtu(struct net_device *ndev, int mtu)
 	if (ret)
 		goto out;
 
+	num_chn = nvdev->num_chn;
+
 	nvdev->start_remove = true;
 	rndis_filter_device_remove(hdev);
 
@@ -883,7 +886,7 @@ static int netvsc_change_mtu(struct net_device *ndev, int mtu)
 
 	memset(&device_info, 0, sizeof(device_info));
 	device_info.ring_size = ring_size;
-	device_info.num_chn = nvdev->num_chn;
+	device_info.num_chn = num_chn;
 	device_info.max_num_vrss_chns = max_num_vrss_chns;
 	rndis_filter_device_add(hdev, &device_info);