mm/memory: fix missing pte marker for !page on pte zaps

Commit 0cf18e83 of large folio zap work broke uffd-wp.  Now mm's uffd
unit test "wp-unpopulated" will trigger this WARN_ON_ONCE().

The WARN_ON_ONCE() asserts that an VMA cannot be registered with
userfaultfd-wp if it contains a !normal page, but it's actually possible. 
One example is an anonymous vma, register with uffd-wp, read anything will
install a zero page.  Then when zap on it, this should trigger.

What's more, removing that WARN_ON_ONCE may not be enough either, because
we should also not rely on "whether it's a normal page" to decide whether
pte marker is needed.  For example, one can register wr-protect over some
DAX regions to track writes when UFFD_FEATURE_WP_ASYNC enabled, in which
case it can have page==NULL for a devmap but we may want to keep the
marker around.

Link: https://lkml.kernel.org/r/20240313213107.235067-1-peterx@redhat.com
Fixes: 0cf18e83 ("mm/memory: handle !page case in zap_present_pte() separately")
Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -1536,7 +1536,9 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
 		ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
 		arch_check_zapped_pte(vma, ptent);
 		tlb_remove_tlb_entry(tlb, pte, addr);
-		VM_WARN_ON_ONCE(userfaultfd_wp(vma));
+		if (userfaultfd_pte_wp(vma, ptent))
+			zap_install_uffd_wp_if_needed(vma, addr, pte, 1,
+						      details, ptent);
 		ksm_might_unmap_zero_page(mm, ptent);
 		return 1;
 	}