kernel/vm: check if page is in area

Checks if a `vm_page` is part of a `VMArea` before doing work with
it, as pages in a `VMCache` that an area is a part of might not
belong to that area.

This fixes a bug for copy-on-write areas when an application is
`fork`ing.

Change-Id: Ic5683c67865b41bf3708bb7ea4104502ddf31a19
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6496
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Reviewed-by: Axel Dörfler <axeld@pinc-software.de>

kernel/vm: handle page protections in cut_area

- Resize the `page_protections` array in `cut_area` and also shift
the bits if necessary.
- Set the correct protection array as well as the real page
protections for the second area produced by `cut_area`.

Change-Id: I62293480487e869420ebe5a3bc729cec2a14c687
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6395
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

Revert "kernel/vm: handle page protections in cut_area"

This reverts commit de07bc3fa58600e923d75cb732c60dacf160c098.

That's what I get for fixing bugs on test branches.

kernel/vm: handle page protections in cut_area

- Resize the `page_protections` array in `cut_area` and also shift
the bits if necessary.
- Set the correct protection array as well as the real page
protections for the second area produced by `cut_area`.

Change-Id: I62293480487e828970ebe5a3bc729cec2a14c687

tests: Add mmap_cut_tests.

Tests #17556 and #16936. Loosely based on X512's example program.