netdev: Initialize at the beginning of netdev_unregister_provider().

Otherwise, if netdev_unregister_provider() is called before any other
netdev function, netdev_class_mutex is not initialized and the attempt to
lock it aborts.

This doesn't fix an existing bug but with the following commit
--enable-dummy=system will make netdev_unregister_provider() the first
netdev function to be called.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>

lib/netdev.c

@@ -257,6 +257,8 @@ netdev_unregister_provider(const char *type)
     struct netdev_registered_class *rc;
     int error;
 
+    netdev_initialize();
+
     ovs_mutex_lock(&netdev_class_mutex);
     rc = netdev_lookup_class(type);
     if (!rc) {