LU-12780 osp: don't use ptlrpc_thread for opd_update_thread

LU-12780 osp: don't use ptlrpc_thread for opd_update_thread

rather than ptlrpc_thread, use native kthreads functionality.

- There is no need to synchornized on startup - do all the startup
  that can fail before starting the thread.  This involves puting the
  lu_env in the per-thread struct osp_updates.

- Synchronization on shutdown is down with kthread_stop() and
  kthread_should_stop().  lu_env_put() call is moved to after
  kthread_stop() call, as it is theoretically possible that the
  thread function never gets called, so it isn't safe for it to
  be responsible for cleanup.

- opd_update_thread is replace with ou_update_task in struct
  osp_updates.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I78dafbadf419ee9b80a9bd0046152fb6f293f191
Reviewed-on: https://review.whamcloud.com/36264
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>