net: Fix a data-race around netdev_unregister_timeout_secs.

While reading netdev_unregister_timeout_secs, it can be changed
concurrently.  Thus, we need to add READ_ONCE() to its reader.

Fixes: 5aa3afe1

 ("net: make unregister netdev warning timeout configurable")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>