[IPV6]: Introduce ip6_dst_idev() to get inet6_dev{} stored in dst_entry{}.

Otherwise, we will see a lot of casts...

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
index e4438de..f9cde44 100644
--- a/include/net/ip6_fib.h
+++ b/include/net/ip6_fib.h
@@ -107,6 +107,11 @@
 	u8				rt6i_protocol;
 };
 
+static inline struct inet6_dev *ip6_dst_idev(struct dst_entry *dst)
+{
+	return ((struct rt6_info *)dst)->rt6i_idev;
+}
+
 struct fib6_walker_t
 {
 	struct fib6_walker_t *prev, *next;