LoongArch: Export symbol invalid_pud_table for modules building

Export symbol invalid_pud_table for modules building (such as the KVM
module) if 4-level page tables enabled. Otherwise we get:

ERROR: modpost: "invalid_pud_table" [arch/loongarch/kvm/kvm.ko] undefined!

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Tianrui Zhao <zhaotianrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>