9p: Use kthread_stop instead of sending a SIGKILL.

Since the kthread api does not bump the reference count on
processes that tracked it is not safe allow user space to
kill the threads, as I still retain a pointer to the task_struct.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>