bridge: vlan: use br_vlan_should_use to simplify __vlan_add/del

The checks that lead to num_vlans change are always what
br_vlan_should_use checks for, namely if the vlan is only a context or
not and depending on that it's either not counted or counted
as a real/used vlan respectively.
Also give better explanation in br_vlan_should_use's comment.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index 4ed8308..1ff6a0f 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -400,7 +400,7 @@
 	return v->flags & BRIDGE_VLAN_INFO_BRENTRY;
 }
 
-/* check if we should use the vlan entry is usable */
+/* check if we should use the vlan entry, returns false if it's only context */
 static inline bool br_vlan_should_use(const struct net_bridge_vlan *v)
 {
 	if (br_vlan_is_master(v)) {