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>