LoongArch: Code improvements in function pcpu_populate_pte()

Do some code improvements in function pcpu_populate_pte():
1. Add memory allocation failure handling;
2. Replace pgd_populate() with p4d_populate(), it will be useful if
   there are four-level page tables.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>