riscv: Ensure page table writes are flushed when initializing KASAN vmalloc

Make sure that writes to kernel page table during KASAN vmalloc
initialization are made visible by adding a sfence.vma.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Fixes: e178d670

 ("riscv/kasan: add KASAN_VMALLOC support")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>