[NET]: Take RTNL when unregistering notifier

The netdev notifier call chain is currently unregistered without taking
any locks outside the notifier system.  Because the notifier system itself
does not synchronise unregistration with respect to the calling of the
chain, we as its user need to do our own locking.

We are supposed to take the RTNL for all calls to netdev notifiers, so
taking the RTNL should be sufficient to protect it.

The registration path in dev.c already takes the RTNL so it's OK.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>