net: call newid/getid without rtnl mutex held

Both functions take nsid_lock and don't rely on rtnl lock.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>