dtrace: do not overrun the start of the user stack

dtrace: do not overrun the start of the user stack

When scanning user stacks in dtrace_getufpstack(), we iterate from the
current stack pointer back to the start of the stack, getting the
unsigned long at each location and seeing if we can interpret it as a
pointer.

However, since the stack grows down on all platforms supported by
DTrace, the 'start' of the stack is the end of the VMA -- so we should
stop one unsigned long before the beginning, or we'll try to read off
the end (harmlessly, but still.)

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