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: NKuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: NDmitry Vyukov <dvyukov@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>