mm: use a folio in copy_present_pte()

mm: use a folio in copy_present_pte()

We still have to keep the page around because we need to know which page
in the folio we're copying, but we can replace five implict calls to
compound_head() with one.

Link: https://lkml.kernel.org/r/20230116191813.2145215-6-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>