riscv: mm: Update mmap_rnd_bits_max

riscv: mm: Update mmap_rnd_bits_max

ARCH_MMAP_RND_BITS_MAX is based on Sv39, which leaves a few
potential bits of mmap randomness on the table if we end up enabling
4/5-level paging. Update mmap_rnd_bits_max to take the final address
space size into account. This increases mmap_rnd_bits_max from 24 to
33 with Sv48/57.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20230929211155.3910949-6-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>