net: introduce nla_policy for IFLA_NEW_IFINDEX

In this case, we don't need to check that new_ifindex is positive in
validate_linkmsg.

Fixes: eeb85a14 ("net: Allow to specify ifindex when device is moved to another namespace")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -1877,6 +1877,7 @@ static const struct nla_policy ifla_policy[IFLA_MAX+1] = {
 				    .len = ALTIFNAMSIZ - 1 },
 	[IFLA_PERM_ADDRESS]	= { .type = NLA_REJECT },
 	[IFLA_PROTO_DOWN_REASON] = { .type = NLA_NESTED },
+	[IFLA_NEW_IFINDEX]	= NLA_POLICY_MIN(NLA_S32, 1),
 };
 
 static const struct nla_policy ifla_info_policy[IFLA_INFO_MAX+1] = {
@@ -2266,9 +2267,6 @@ static int validate_linkmsg(struct net_device *dev, struct nlattr *tb[])
 			return -EINVAL;
 	}
 
-	if (tb[IFLA_NEW_IFINDEX] && nla_get_s32(tb[IFLA_NEW_IFINDEX]) <= 0)
-		return -EINVAL;
-
 	if (tb[IFLA_AF_SPEC]) {
 		struct nlattr *af;
 		int rem, err;