use kthread_destroy_worker to destroy workers and
release their associated kthreads.

changes in v3:
	- introduced in the series
changes in v4:
	- none
changes in v5:
	- none
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>