xfrm: fix xfrm_dev_event() missing when compile without CONFIG_XFRM_OFFLOAD

In commit d77e38e612a0 ("xfrm: Add an IPsec hardware offloading API") we
make xfrm_device.o only compiled when enable option CONFIG_XFRM_OFFLOAD.
But this will make xfrm_dev_event() missing if we only enable default XFRM
options.

Then if we set down and unregister an interface with IPsec on it. there
will no xfrm_garbage_collect(), which will cause dev usage count hold and
get error like:

unregister_netdevice: waiting for <dev> to become free. Usage count = 4

Fixes: d77e38e612a0 ("xfrm: Add an IPsec hardware offloading API")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 7e7e2b0..62f5a25 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -1850,8 +1850,9 @@
 }
 #endif
 
-#ifdef CONFIG_XFRM_OFFLOAD
 void __net_init xfrm_dev_init(void);
+
+#ifdef CONFIG_XFRM_OFFLOAD
 int validate_xmit_xfrm(struct sk_buff *skb, netdev_features_t features);
 int xfrm_dev_state_add(struct net *net, struct xfrm_state *x,
 		       struct xfrm_user_offload *xuo);
@@ -1877,10 +1878,6 @@
 	}
 }
 #else
-static inline void __net_init xfrm_dev_init(void)
-{
-}
-
 static inline int validate_xmit_xfrm(struct sk_buff *skb, netdev_features_t features)
 {
 	return 0;