ipv6: introduce ip6_rt_put()

As suggested by Eric, we could introduce a helper function
for ipv6 too, to avoid checking if rt is NULL before
dst_release().

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/fib6_rules.c b/net/ipv6/fib6_rules.c
index d9fb911..2e1a432 100644
--- a/net/ipv6/fib6_rules.c
+++ b/net/ipv6/fib6_rules.c
@@ -100,7 +100,7 @@
 		goto out;
 	}
 again:
-	dst_release(&rt->dst);
+	ip6_rt_put(rt);
 	rt = NULL;
 	goto out;