audit: cleanup prune_tree_thread

We can use kthread_run instead of kthread_create+wake_up_process for
creating the thread.

We do not need to set the task state to TASK_RUNNING after schedule(),
the process is in that state already.

And we do not need to set the state to TASK_INTERRUPTIBLE when not
doing schedule() as we set the state to TASK_RUNNING immediately
afterwards.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: <linux-audit@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>