vlan: use xarray iterator to implement /proc/net/vlan/config

Adopt net->dev_by_index as I did in commit 0e0939c0
("net-procfs: use xarray iterator to implement /proc/net/dev")

Not only this removes quadratic behavior, it also makes sure
an existing vlan device is always visible in the dump,
regardless of concurrent net->dev_base_head changes.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240211214404.1882191-2-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>