mm/ksm: use folio in remove_stable_node

mm/ksm: use folio in remove_stable_node

Pages in stable tree are all single normal page, so uses ksm_get_folio()
and folio_set_stable_node(), also saves 3 calls to compound_head().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alex Shi (tencent) <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Cc: Izik Eidus <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Chris Wright <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>