riscv: Fix loading 64-bit NOMMU kernels past the start of RAM

In the Linux kernel, the following vulnerability has been resolved: riscv: Fix loading 64-bit NOMMU kernels past the start of RAM commit 3335068f8721 ("riscv: Use PUD/P4D/PGD pages for the linear mapping") added logic to allow using RAM below the kernel load address. However, this does not work for NOMMU, where PAGE_OFFSET is fixed to the kernel load address. Since that range of memory corresponds to PFNs below ARCH_PFN_OFFSET, mm initialization runs off the beginning of mem_map and corrupts adjacent kernel memory. Fix this by restoring the previous behavior for NOMMU kernels.