mm/page_table_check: fix crash on ZONE_DEVICE

In the Linux kernel, the following vulnerability has been resolved:

mm/page_table_check: fix crash on ZONE_DEVICE

Not all pages may apply to pgtable check. One example is ZONE_DEVICE

pages: they map PFNs directly, and they don't allocate page_ext at all

even if there's struct page around. One may reference

devm_memremap_pages().

When both ZONE_DEVICE and page-table-check enabled, then try to map some

dax memories, one can trigger kernel bug constantly now when the kernel

was trying to inject some pfn maps on the dax device:

kernel BUG at mm/page_table_check.c:55!

While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page

fault resolutions, skip all the checks if page_ext doesn't even exist in

pgtable checker, which applies to ZONE_DEVICE but maybe more.