a protection_max attribute is added in VMArea.
a read-only opened file already can't be mapped shared read-write at the moment,
but can later be changed to read-write with mprotect() or set_area_protection().
When creating the VMArea, the actual maximum protection is stored in the area,
so that it can be checked when needed.
this fixes a VM TODO.
Change-Id: I33b144c192034eeb059f1dede5dbef5af947280d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3804
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
45872f7f9c