riscv: process: fix kernel info leakage

In the Linux kernel, the following vulnerability has been resolved: riscv: process: fix kernel info leakage thread_struct's s[12] may contain random kernel memory content, which may be finally leaked to userspace. This is a security hole. Fix it by clearing the s[12] array in thread_struct when fork. As for kthread case, it's better to clear the s[12] array as well.