netdev-dummy: Make netdev_rx_wait() wakeups work cross-thread for dummies.

netdev-dummy: Make netdev_rx_wait() wakeups work cross-thread for dummies.

Until now, netdev_dummy_rx_wait() has only checked whether the receive
queue for the dummy device is currently empty.  This has worked OK because
in practice packets were queued to dummy devices only from the same thread
that attempted to receive them.  An upcoming commit will use different
threads for these purposes, so this commit switches to a notification
method that works cross-thread.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>