dtrace: ensure we pass a limit to dtrace_stacktrace for stackdepth

dtrace: ensure we pass a limit to dtrace_stacktrace for stackdepth

When determining the (kernel) stackdepth, we pass scratch memory to the
dtrace_stacktrace() function because we are not interested in the actual
program counter values.  However, we were passing in 0 as limit rather
than the actual maximum number of PCs that could fit in the remaining
scratch memory space.

We now also add no-fault protection to dtrace_getstackdepth().

Orabug: 25559321
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>