LU-14428 libcfs: simplify task management in tracefile.c

LU-14428 libcfs: simplify task management in tracefile.c

The waitqueue, mutex, and two completions are not needed.
We can use kthread_stop/kthread_should_stop to synchronize
shutdown, cmpxchg() to ensure only one task is started, and a simple
wake_up_process() to wake the process.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I45390c2cb58b09a85033a7006412a0e2033130c5
Reviewed-on: https://review.whamcloud.com/41492
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>