x86: don't make copies of shared pages in VirtualAddressSpace::clone, as VirtualAddressSpace::revertToKernelAddressSpace will not free shared pages.

We no longer leak enormous amounts of memory on fork(). As an added bonus,
fork() is also much faster now.
x86: extend kernel mapping check in VirtualAddressSpace::revertToKernelAddressSpace to check the physical page as well.