riscv: fix pfn_to_virt err in do_page_fault().

The argument to pfn_to_virt() should be pfn not the value of CSR_SATP.

Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: liush <liush@allwinnertech.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>