net: make sure we never create ifindex = 0

Instead of allocating from 1 use proper xa_init flag,
to protect ourselves from IDs wrapping back to 0.

Fixes: 759ab1ed ("net: store netdevs in an xarray")
Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Link: https://lore.kernel.org/all/20230728162350.2a6d4979@hermes.local/Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230731171159.988962-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -11271,8 +11271,7 @@ static int __net_init netdev_init(struct net *net)
 	if (net->dev_index_head == NULL)
 		goto err_idx;
 
-	net->ifindex = 1;
-	xa_init_flags(&net->dev_by_index, XA_FLAGS_ALLOC);
+	xa_init_flags(&net->dev_by_index, XA_FLAGS_ALLOC1);
 
 	RAW_INIT_NOTIFIER_HEAD(&net->netdev_chain);