dtrace: armour ustack() against kernel threads, !task->mm, and corrupt usersp.

dtrace: armour ustack() against kernel threads, !task->mm, and corrupt usersp.

Kernel threads have no userspace stack, by definition: we should not assume they
do.  Further, tasks with no mm (whether because they are kernel threads or for
any other reason) should not be ustack()ed, nor tasks in which find_vma() cannot
find the vma corresponding to the usersp.  (Possible causes for this might be a
task which just smashed its own userspace sp or a task which is in the middle of
exiting or exec()ing.)

Orabug: 17591351

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>