dtrace: use copy_from_user() when walking userspace stacks

dtrace: use copy_from_user() when walking userspace stacks

We were using get_user(), but that doesn't reliably work on all
platforms (such as SPARC64) and cannot trap faults, which meant we were
jumping through extra hoops to trap faults when copy_from_user() does
that anyway.

The extra copy is notably less efficient (since we end up looping over,
and copying, essentially the entire user stack in one-word increments),
but has the advantage of actually working.

Orabug: 22629102
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Acked-by: Kris Van Hees <kris.van.hees@oracle.com>