ref_tracker: implement use-after-free detection

Whenever ref_tracker_dir_init() is called, mark the struct ref_tracker_dir
as dead.

Test the dead status from ref_tracker_alloc() and ref_tracker_free()

This should detect buggy dev_put()/dev_hold() happening too late
in netdevice dismantle process.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>