mm: Change mmap_rnd_bits_max to __ro_after_init

mm: Change mmap_rnd_bits_max to __ro_after_init

Allow mmap_rnd_bits_max to be updated on architectures that
determine virtual address space size at runtime instead of relying
on Kconfig options by changing it from const to __ro_after_init.

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/20230929211155.3910949-5-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>