net device are refcounted. Over the years we had numerous bugs
caused by imbalanced dev_hold() and dev_put() calls.

The general idea is to be able to precisely pair each decrement with
a corresponding prior increment. Both share a cookie, basically
a pointer to private data storing stack traces.

This patch adds dev_hold_track() and dev_put_track().

To use these helpers, each data structure owning a refcount
should also use a "netdevice_tracker" to pair the hold and put.

netdevice_tracker dev_tracker;
...
dev_hold_track(dev, &dev_tracker, GFP_ATOMIC);
...
dev_put_track(dev, &dev_tracker);

Whenever a leak happens, we will get precise stack traces
of the point dev_hold_track() happened, at device dismantle phase.

We will also get a stack trace if too many dev_put_track() for the same
netdevice_tracker are attempted.

This is guarded by CONFIG_NET_DEV_REFCNT_TRACKER option.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>