mm: define general function pXd_init()

pud_init(), pmd_init() and kernel_pte_init() are duplicated defined in
file kasan.c and sparse-vmemmap.c as weak functions.  Move them to generic
header file pgtable.h, architecture can redefine them.

Link: https://lkml.kernel.org/r/20241104070712.52902-1-maobibo@loongson.cn


Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: WANG Xuerui <kernel@xen0n.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>