LoongArch: Use accessors to page table entries instead of direct dereference

As very well explained in commit 20a004e7 ("arm64: mm: Use
READ_ONCE/WRITE_ONCE when accessing page tables"), an architecture whose
page table walker can modify the PTE in parallel must use READ_ONCE()/
WRITE_ONCE() macro to avoid any compiler transformation.

So apply that to LoongArch which is such an architecture, in order to
avoid potential problems.

Similar to commit edf95564 ("riscv: Use accessors to page table
entries instead of direct dereference").

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