LoongArch: Fix copy_thread() build errors

Commit c5febea0 ("fork: Pass struct kernel_clone_args into
copy_thread") change the prototype of copy_thread(), while commit
5bd2e97c ("fork: Generalize PF_IO_WORKER handling") change
the structure of kernel_clone_args. They cause build errors, so fix it.

Fixes: 5bd2e97c ("fork: Generalize PF_IO_WORKER handling")
Fixes: c5febea0 ("fork: Pass struct kernel_clone_args into copy_thread")
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>