[PATCH v2] riscv: Make mmap() with PROT_WRITE imply PROT_READ

[SS JieJi]

The v2 patch looks great,
> -       if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
> -               return -EINVAL;
> -
This also removes the check for --x pages, which used to be present in
previous versions (before the submission of the to-be-reverted patch).
Is this intended? Thanks!