vlan: Fix build error wth vlan_get_encap_level()

The new function vlan_get_encap_level() uses vlan_dev_priv()
which is only conditionally avaialble when VLAN support is
enabled. Make vlan_get_encap_level() conditionally available
as well.

Fixes: 44a4085538c8 ("bonding: Fix stacked device detection in arp monitoring")
Reported-by: Stephen Rothwell <[e-mail]>
CC: Stephen Rothwell <[e-mail]>
Signed-off-by: Vlad Yasevich <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/if_vlan.h

···
 				 const struct net_device *by_dev);
 
 extern bool vlan_uses_dev(const struct net_device *dev);
+
+static inline int vlan_get_encap_level(struct net_device *dev)
+{
+	BUG_ON(!is_vlan_dev(dev));
+	return vlan_dev_priv(dev)->nest_level;
+}
 #else
 static inline struct net_device *
 __vlan_find_dev_deep(struct net_device *real_dev,
···
 static inline bool vlan_uses_dev(const struct net_device *dev)
 {
 	return false;
+}
+static inline int vlan_get_encap_level(struct net_device *dev)
+{
+	BUG();
+	return 0;
 }
 #endif
 
···
 		skb->protocol = htons(ETH_P_802_2);
 }
 
-static inline int vlan_get_encap_level(struct net_device *dev)
-{
-	BUG_ON(!is_vlan_dev(dev));
-	return vlan_dev_priv(dev)->nest_level;
-}
 #endif /* !(_LINUX_IF_VLAN_H_) */