`mmap` breaks easily

This looks like an ok fix for the issue at hand to me. However, I think `mmap` has a bunch of problems and this is more of a band-aid solution. For example, this implementaiton breaks if the affected area contains both pages mapped with `PROT_NONE` and pages with some access allowed. From my reading of the man pages, it is perfectly fine to use linux `mmap` like that.

For a proper solution, we should decide on which level `mmap` should operate on:

If it is a high level api, we should track the state of all pages mapped via `mmap`. `munmap` should refuse to unmap pages which have not been allocated via `mmap`. This could be done via a hashtable or using the extra bits available in the pagetable. As an example of why this is needed, you could use `munmap` to unmap the identity mappings with the current implementation.

If it is a low level api, it should interact directly with the free lists and the page table. In that case, `munmap` should just walk the page table and add anything it finds to the physical free list. We should specify incorrect use of `mmap` to be UB, as we can not really detect it reliably. This would make our mmap incompatible with the linux one.

I would prefer `mmap` to be an optional high level feature and to have an additional set of syscalls that allows direct access to the free lists.

_Originally posted by @m-mueller678 in https://github.com/hermit-os/kernel/issues/2008#issuecomment-3432646702_