LoongArch: Fix arch_dup_task_struct() for CONFIG_RANDSTRUCT

Now the optimized version of arch_dup_task_struct() for LoongArch
assumes 'thread' is the last member of 'task_struct'. But this is
not true if CONFIG_RANDSTRUCT is enabled after Linux-6.16.

So fix the arch_dup_task_struct() function for CONFIG_RANDSTRUCT by
copying the whole 'task_struct'.

Cc: stable@vger.kernel.org   # 6.16+
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: chenhuacai

arch/loongarch/kernel/process.c

@@ -130,6 +130,11 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 
 	preempt_enable();
 
+	if (IS_ENABLED(CONFIG_RANDSTRUCT)) {
+		memcpy(dst, src, sizeof(struct task_struct));
+		return 0;
+	}
+
 	if (!used_math())
 		memcpy(dst, src, offsetof(struct task_struct, thread.fpu.fpr));
 	else