Update the thread shim to use the current kernel threading API.

Update the thread shim to use the current kernel threading API.
We need to use kthread_create() here for a few reasons.  First
off to old kernel_thread() API functioin will be going away.
Secondly, and more importantly if I use kthread_create() we can
then properly implement a thread_exit() function which terminates
the kernel thread at any point with do_exit().  This fixes our
cleanup bug which was caused by dropping a mutex twice after
thread_exit() didn't really exit.

git-svn-id: https://outreach.scidac.gov/svn/spl/trunk@66 7e1ea52c-4ff2-0310-8f11-9dd32ca42a1c