Unmap pages from the kernel linear mapping after free_pages().
This results in a large slowdown, but helps to find certain types
of memory corruption.

For architectures which don't enable ARCH_SUPPORTS_DEBUG_PAGEALLOC,
fill the pages with poison patterns after free_pages() and verify
the patterns before alloc_pages(). Additionally,
this option cannot be enabled in combination with hibernation as
that would result in incorrect warnings of memory corruption after
a resume because free pages are not saved to the suspend image.