LoongArch: mm: Avoid unnecessary page fault retires on shared memory types

Commit d9272525 ("mm: avoid unnecessary page fault retires on
shared memory types") modifies do_page_fault() to handle the VM_FAULT_
COMPLETED case, but forget to change for LoongArch, so fix it as other
architectures does.

Fixes: d9272525 ("mm: avoid unnecessary page fault retires on shared memory types")
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>