netdev: Initialize netdev_class_mutex.

This code path currently does not initialize
netdev_class_mutex.
dummy_enable
 ->netdev_dummy_register
   ->netdev_register_provider
     ->ovs_mutex_lock(&netdev_class_mutex)

ovsdb-server on windows crashes without it.

This commit adds a new initialization function.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
ofproto: Don't destroy mutex before its use.

Currently, we are calling guarded_list_destroy()
to destroy a mutex and then go ahead and use it through
delete_group
 ->delete_group__
   ->handle_flow_mod__
    ->run_rule_executes
     ->guarded_list_pop_all

The group related unit tests cause ovs-vswitchd to crash
because of this (on windows).

Calling guarded_list_destroy() after delete_group() solves the
problem.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>