userfaultfd: remove WRITE_ONCE when setting folio->index during UFFDIO_MOVE

When folio is moved with UFFDIO_MOVE it gets locked before the rmap and
index are modified.  Due to the folio lock being already held,
WRITE_ONCE() is not needed when setting the folio index.  Remove it.

Link: https://lkml.kernel.org/r/20240415020821.1152951-1-surenb@google.comReported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -2200,7 +2200,7 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
 		}
 
 		folio_move_anon_rmap(src_folio, dst_vma);
-		WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
+		src_folio->index = linear_page_index(dst_vma, dst_addr);
 
 		_dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
 		/* Follow mremap() behavior and treat the entry dirty after the move */

mm/userfaultfd.c

@@ -1026,7 +1026,7 @@ static int move_present_pte(struct mm_struct *mm,
 	}
 
 	folio_move_anon_rmap(src_folio, dst_vma);
-	WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
+	src_folio->index = linear_page_index(dst_vma, dst_addr);
 
 	orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
 	/* Follow mremap() behavior and treat the entry dirty after the move */