netdev: Make netdev_from_name() take a reference to its returned netdev.

This API change is necessary for thread safety, to be added in an upcoming
commit.  Otherwise, the client would not be able to safely use the returned
netdev because it could already have been destroyed.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>