LoongArch: Move {dmw,tlb}_virt_to_page() definition to page.h

These two functions are implemented in pgtable.c, and they are needed
only by the virt_to_page() macro in page.h. Having the prototypes in
pgtable.h causes a circular dependency between page.h and pgtable.h,
because the virt_to_page() macro in page.h needs pgtable.h for these
two functions, while pgtable.h needs various definitions from page.h
(e.g. pte_t and pgt_t).

Let's avoid this circular dependency by moving the function prototypes
to page.h.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>