[IPV6]: Fix ipv6 fragment ID selection at slow path

Signed-Off-By: Yan Zheng <yanzheng@21cn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -666,7 +666,7 @@ slow_path:
 		 */
 		fh->nexthdr = nexthdr;
 		fh->reserved = 0;
-		if (frag_id) {
+		if (!frag_id) {
 			ipv6_select_ident(skb, fh);
 			frag_id = fh->identification;
 		} else