mm: Move lowmem_page_address() a little later

LoongArch will override page_to_virt() which use page_address() in the
KFENCE case (by defining WANT_PAGE_VIRTUAL/HASHED_PAGE_VIRTUAL). So move
lowmem_page_address() a little later to avoid such build errors:

error: implicit declaration of function 'page_address'.

Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>