xfrm: Fix warning in xfrm6_tunnel_net_exit.

We need to make sure that all states are really deleted
before we check that the state lists are empty. Otherwise
we trigger a warning.

Fixes: baeb0dbbb5659 ("xfrm6_tunnel: exit_net cleanup check added")
Reported-and-tested-by:syzbot+777bf170a89e7b326405@syzkaller.appspotmail.com
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index a872379..45e75c3 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -375,6 +375,7 @@
 int xfrm_input_register_afinfo(const struct xfrm_input_afinfo *afinfo);
 int xfrm_input_unregister_afinfo(const struct xfrm_input_afinfo *afinfo);
 
+void xfrm_flush_gc(void);
 void xfrm_state_delete_tunnel(struct xfrm_state *x);
 
 struct xfrm_type {