This makes no functional difference, but it makes it clear that
MemoryManager and PhysicalRegion take over the actual physical
page represented by this PhysicalPage instance.
Pages created with PhysicalPage::create_eternal() should *not* be
returnable to the freelist; and pages created with the regular
PhysicalPage::create() should be; not the other way around.
This significantly reduces the pressure on the kernel heap when
allocating a lot of pages.
Previously at about 250MB allocated, the free page list would outgrow
the kernel's heap. Given that there is no longer a page list, this does
not happen.
The next barrier will be the kernel memory used by the page records for
in-use memory. This kicks in at about 1GB.
