lightnvm: pblk: delete writer kick timer before stopping thread

Unless we delete the timer that wakes up the write thread
before we stop the thread we risk re-starting the thread, so
delete the timer first.

Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Reviewed-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>