[IPv6]: use container_of() macro in fib6_clean_node()

In ip6_fib.c, fib6_clean_node() casts a fib6_walker_t pointer to
a fib6_cleaner_t pointer assuming a struct fib6_walker_t (field 'w')
is the first field in struct fib6_walker_t.

To prevent any future problems that may occur if one day a field
is inadvertently inserted before the 'w' field in struct fib6_cleaner_t,
(and to improve readability), this patch uses the container_of() macro.
Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -1313,7 +1313,7 @@ static int fib6_clean_node(struct fib6_walker_t *w)
 {
 	int res;
 	struct rt6_info *rt;
-	struct fib6_cleaner_t *c = (struct fib6_cleaner_t*)w;
+	struct fib6_cleaner_t *c = container_of(w, struct fib6_cleaner_t, w);
 
 	for (rt = w->leaf; rt; rt = rt->u.dst.rt6_next) {
 		res = c->func(rt, c->arg);