LoongArch: Fix the !THP build

mainline inclusion
from mainline-v5.19-rc4
commit 501dcbe4
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB


CVE: NA

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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>