mm: move common part of pagetable_*_ctor to helper

Patch series "Account page tables at all levels".

This series should be considered in conjunction with Qi's series [1].
Together, they ensure that page table ctor/dtor are called at all levels
(PTE to PGD) and all architectures, where page tables are regular pages.
Besides the improvement in accounting and general cleanup, this also
create a single place where construction/destruction hooks can be called
for all page tables, namely the now-generic pagetable_dtor() introduced
by Qi, and __pagetable_ctor() introduced in this series.

[1] https://lore.kernel.org/linux-mm/cover.1735549103.git.zhengqi.arch@bytedance.com/


This patch (of 6):

pagetable_*_ctor all have the same basic implementation.  Move the common
part to a helper to reduce duplication.

Link: https://lkml.kernel.org/r/20250103184415.2744423-1-kevin.brodsky@arm.com
Link: https://lkml.kernel.org/r/20250103184415.2744423-2-kevin.brodsky@arm.com


Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>