ipv6: Fix may be used uninitialized warning in rt6_check

rt_cookie might be used uninitialized, fix this by
initializing it.

Fixes: c5cff8561d2d ("ipv6: add rcu grace period before freeing fib6_node")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index a9d3564..48c8c92 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1289,7 +1289,7 @@
 
 static struct dst_entry *rt6_check(struct rt6_info *rt, u32 cookie)
 {
-	u32 rt_cookie;
+	u32 rt_cookie = 0;
 
 	if (!rt6_get_cookie_safe(rt, &rt_cookie) || rt_cookie != cookie)
 		return NULL;