dtrace: do not vmalloc/vfree from probe context

dtrace: do not vmalloc/vfree from probe context

The dtrace_stacktrace() code was using vmalloc/vfree in cases where no
array for PCs was provided (as was the case for getting the stackdepth).
When this code gets called from probe context, we might be processing an
interrupt or trap, and it is not safe to use vmalloc/vfree in that context.
We now pass in a (pre-allocated) scratch buffer instead.

Orabug: 20456889

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Acked-by: Nick Alcock <nick.alcock@oracle.com>