ipv6: ndisc: fix ndisc_send_redirect writing to the wrong skb

Since some refactoring in 5f5a0115, ndisc_send_redirect called
ndisc_fill_redirect_hdr_option on the wrong skb, leading to data corruption or
in the worst case a panic when the skb_put failed.
Signed-off-by: NMatthias Schiffer <mschiffer@universe-factory.net>
Reviewed-by: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -1493,7 +1493,7 @@ void ndisc_send_redirect(struct sk_buff *skb, const struct in6_addr *target)
 	 */
 
 	if (ha)
-		ndisc_fill_addr_option(skb, ND_OPT_TARGET_LL_ADDR, ha);
+		ndisc_fill_addr_option(buff, ND_OPT_TARGET_LL_ADDR, ha);
 
 	/*
 	 *	build redirect option and copy skb over to the new packet.