arm64: kernel thread don't need to save fpsimd context.

kernel thread's default fpsimd state is zero. When fork a thread, if parent is kernel thread,
 and save hardware context to parent's fpsimd state, but this hardware context is user
process's context, because kernel thread don't use fpsimd, it will not introduce issue,
it add a little cost.
Signed-off-by: NJanet Liu <janet.liu@spreadtrum.com>
Signed-off-by: NChunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/process.c

@@ -233,7 +233,8 @@ void release_thread(struct task_struct *dead_task)
 
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 {
-	fpsimd_preserve_current_state();
+	if (current->mm)
+		fpsimd_preserve_current_state();
 	*dst = *src;
 	return 0;
 }