mm/ksm: use folio in write_protect_page

Compound page is checked and skipped before write_protect_page() called,
use folio to save a few compound_head checks.

Link: https://lkml.kernel.org/r/20240411061713.1847574-8-alexs@kernel.org


Signed-off-by: Alex Shi (tencent) <alexs@kernel.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Izik Eidus <izik.eidus@ravellosystems.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>