riscv: Fixup pfn_valid error with wrong max_mapnr

The max_mapnr is the number of PFNs, not absolute PFN offset.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Fixes: d0d8aae6 ("RISC-V: Set maximum number of mapped pages correctly")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>