LU-9859 libcfs: use wait_event_timeout() in tracefiled().

LU-9859 libcfs: use wait_event_timeout() in tracefiled().

By using wait_event_timeout() we can make it more clear what is being
waited for, and when the loop terminates.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I5d015a3cbdbb342a5117e2c328680b3ec13aeb58
Reviewed-on: https://review.whamcloud.com/39293
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>