This method is used in BXVGADevice to create pages for the framebuffer;
we should neither make the PhysicalPage instances eternal, nor hand over
actual physical pages to the memory allocator.
This patch moves away from using kmalloc memory for thread kernel stacks.
This reduces pressure on kmalloc (16 KB per thread adds up fast) and
prevents kernel stack overflow from scribbling all over random unrelated
kernel memory.
