vlan: support "loose binding" to the underlying network device

Currently the UP/DOWN state of VLANs is synchronized to the state of the
underlying device, meaning all VLANs are set down once the underlying
device is set down. This causes all routes to the VLAN devices to vanish.

Add a flag to specify a "loose binding" mode, in which only the operstate
is transfered, but the VLAN device state is independant.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
index 1483243..225aa2f 100644
--- a/net/8021q/vlan.c
+++ b/net/8021q/vlan.c
@@ -431,6 +431,7 @@
 	struct vlan_group *grp;
 	int i, flgs;
 	struct net_device *vlandev;
+	struct vlan_dev_info *vlan;
 	LIST_HEAD(list);
 
 	if (is_vlan_dev(dev))
@@ -507,7 +508,9 @@
 			if (!(flgs & IFF_UP))
 				continue;
 
-			dev_change_flags(vlandev, flgs & ~IFF_UP);
+			vlan = vlan_dev_info(vlandev);
+			if (!(vlan->flags & VLAN_FLAG_LOOSE_BINDING))
+				dev_change_flags(vlandev, flgs & ~IFF_UP);
 			vlan_transfer_operstate(dev, vlandev);
 		}
 		break;
@@ -523,7 +526,9 @@
 			if (flgs & IFF_UP)
 				continue;
 
-			dev_change_flags(vlandev, flgs | IFF_UP);
+			vlan = vlan_dev_info(vlandev);
+			if (!(vlan->flags & VLAN_FLAG_LOOSE_BINDING))
+				dev_change_flags(vlandev, flgs | IFF_UP);
 			vlan_transfer_operstate(dev, vlandev);
 		}
 		break;