netdev-dummy: Fix synchronization error in netdev_dummy_get_config().

Found by Clang.

Signed-off-by: Ben Pfaff <blp@nicira.com>
netdev-dummy: Add Clang lock annotations.

The locks and unlocks added in this commit are superfluous but should not
hurt performance and make Clang happy.

Signed-off-by: Ben Pfaff <blp@nicira.com>