net-procfs: use xarray iterator to implement /proc/net/dev

net-procfs: use xarray iterator to implement /proc/net/dev

In commit 759ab1edb56c ("net: store netdevs in an xarray")
Jakub added net->dev_by_index to map ifindex to netdevices.

We can get rid of the old hash table (net->dev_index_head),
one patch at a time, if performance is acceptable.

This patch removes unpleasant code to something more readable.

As a bonus, /proc/net/dev gets netdevices sorted by their ifindex.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240207165318.3814525-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>