x86/process: Clone FPU in copy_thread()

There is no reason to clone FPU in arch_dup_task_struct(). Quite the
contrary - it prevents optimizations. Move it to copy_thread().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.780714235@linutronix.de