riscv: mm: Fixup spurious fault of kernel vaddr

RISC-V specification permits the caching of PTEs whose V (Valid)
bit is clear. Operating systems must be written to cope with this
possibility, but implementers are reminded that eagerly caching
invalid PTEs will reduce performance by causing additional page
faults.

So we must keep vmalloc_fault for the spurious page faults of kernel
virtual address from an OoO machine.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>