kthread: save thread function

It's handy to keep the kthread_fn just as a unique cookie to identify
classes of kthreads.  E.g. if you can verify that a given task is
running your thread_fn, then you may know what sort of type kthread_data
points to.

We'll use this in nfsd to pass some information into the vfs.  Note it
will need kthread_data() exported too.

Original-patch-by: Tejun Heo <tj@kernel.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>