s390/unwind: correct stack switching during unwind

Adjust conditions in on_stack function. That fixes backchain unwinder
which was unable to read pt_regs at the very bottom of the stack and
hence couldn't follow stacks (e.g. from async stack to a task stack).

Fixes: 78c98f90 ("s390/unwind: introduce stack unwind API")
Reported-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/include/asm/stacktrace.h

@@ -30,7 +30,7 @@ static inline bool on_stack(struct stack_info *info,
 		return false;
 	if (addr + len < addr)
 		return false;
-	return addr >= info->begin && addr + len < info->end;
+	return addr >= info->begin && addr + len <= info->end;
 }
 
 static inline unsigned long get_stack_pointer(struct task_struct *task,