When one macvlan device is dismantled, we can avoid one
synchronize_rcu() call done after deletion from hash list, since caller
will perform a synchronize_net() call after its ndo_stop() call.

Add a new netdev->dismantle field to signal this dismantle intent.

Reduces RTNL hold time.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Patrick McHardy <kaber@trash.net>
CC: Ben Greear <greearb@candelatech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>