x86/fpu: Add XFD state to fpstate

Add storage for XFD register state to struct fpstate. This will be used to
store the XFD MSR state. This will be used for switching the XFD MSR when
FPU content is restored.

Add a per-CPU variable to cache the current MSR value so the MSR has only
to be written when the values are different.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211021225527.10184-15-chang.seok.bae@intel.com