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>

arch/loongarch/mm/init.c

@@ -240,6 +240,7 @@ pgd_t swapper_pg_dir[_PTRS_PER_PGD] __section(".bss..swapper_pg_dir");
 pgd_t invalid_pg_dir[_PTRS_PER_PGD] __page_aligned_bss;
 #ifndef __PAGETABLE_PUD_FOLDED
 pud_t invalid_pud_table[PTRS_PER_PUD] __page_aligned_bss;
+EXPORT_SYMBOL(invalid_pud_table);
 #endif
 #ifndef __PAGETABLE_PMD_FOLDED
 pmd_t invalid_pmd_table[PTRS_PER_PMD] __page_aligned_bss;