LoongArch: Fix the !THP build

Fix the !THP build by making pmd_pfn() available in all configurations.
Because pmd_pfn() is used in mm/page_vma_mapped.c whether or not THP is
configured.
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/pgtable.h

@@ -426,6 +426,11 @@ static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,
 
 #define kern_addr_valid(addr)	(1)
 
+static inline unsigned long pmd_pfn(pmd_t pmd)
+{
+	return (pmd_val(pmd) & _PFN_MASK) >> _PFN_SHIFT;
+}
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 
 /* We don't have hardware dirty/accessed bits, generic_pmdp_establish is fine.*/
@@ -497,11 +502,6 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)
 	return pmd;
 }
 
-static inline unsigned long pmd_pfn(pmd_t pmd)
-{
-	return (pmd_val(pmd) & _PFN_MASK) >> _PFN_SHIFT;
-}
-
 static inline struct page *pmd_page(pmd_t pmd)
 {
 	if (pmd_trans_huge(pmd))