riscv: mm: Unconditionally sfence.vma for spurious fault

In the Linux kernel, the following vulnerability has been resolved: riscv: mm: Unconditionally sfence.vma for spurious fault Svvptc does not guarantee that it's safe to just return here. Since we have already cleared our bit, if, theoretically, the bounded timeframe for the accessed page to become valid still hasn't happened after sret, we could fault again and actually crash. Hopefully, these spurious faults should be rare enough that this is an acceptable slowdown.