x86/fpu: Replace KVMs home brewed FPU copy to user

Similar to the copy from user function the FPU core has this already
implemented with all bells and whistles.

Get rid of the duplicated code and use the core functionality.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: kvm@vger.kernel.org
Link: https://lkml.kernel.org/r/20211015011539.244101845@linutronix.de