The situation below would cause a kernel crash on any earlier 3.x
release, with ktask implemented in a dynamically loaded/unloaded
module:

CPU0:  rtdm_task_destroy(ktask)
       ...
       rmmod(module)

CPU1:  ktask()
       ...
       ...
       __xnthread_test_cancel()
           do_exit()
              (last) schedule()
	         OOPS: prev still treading on stale memory

In this case, the module would be unmapped too early, before the
cancelled task can ultimately schedule away.

The changes also fix a stale reference from the joiner thread to the
former ->idtag field, after the joinee's TCB has been dropped.