iplink: do not require assigning negative ifindex at link creation

Since commit 3c682146, iplink requires assigning negative
ifindex (-1) to the kernel when creating interface without
specifying index.

v2: checking whether index is -1, suggested by Cong Wang.

Cc: Cong Wang <cwang@twopensource.com>
Signed-off-by: Atzm Watanabe <atzm@stratosphere.co.jp>
Acked-by: Cong Wang <cwang@twopensource.com>

ip/iplink.c

@@ -689,6 +689,9 @@ static int iplink_modify(int cmd, unsigned int flags, int argc, char **argv)
 			addattr_l(&req.n, sizeof(req), IFLA_LINK, &ifindex, 4);
 		}
 
+		if (index == -1)
+			req.i.ifi_index = 0;
+		else
 			req.i.ifi_index = index;
 	}