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.