mm: page_table_check: Ensure user pages are not slab pages

The current uses of PageAnon in page table check functions can lead to
type confusion bugs between struct page and slab [1], if slab pages are
accidentally mapped into the user space. This is because slab reuses the
bits in struct page to store its internal states, which renders PageAnon
ineffective on slab pages.

Since slab pages are not expected to be mapped into the user space, this
patch adds BUG_ON(PageSlab(page)) checks to make sure that slab pages
are not inadvertently mapped. Otherwise, there must be some bugs in the
kernel.

Reported-by: syzbot+fcf1a817ceb50935ce99@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/000000000000258e5e05fae79fc1@google.com/ [1]
Fixes: df4e817b7108 ("mm: page table check")
Cc: <stable@vger.kernel.org> # 5.17
Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
Acked-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/r/20230515130958.32471-5-lrh2000@pku.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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