From: Hugh Dickins <hugh@veritas.com>

Remove the support for pte_chain rmap from page table initialization, just
continue to maintain nr_page_table_pages (but only for user page tables -
it also counted vmalloc page tables before, little need, and I'm unsure if
per-cpu stats are safe early enough on all arches).  mm/memory.c is the
only core file affected.

But ppc and ppc64 have found the old rmap page table initialization useful
to support their ptep_test_and_clear_young: so transfer rmap's
initialization to them (even on kernel page tables?  well, okay).