x86/fpu: Restrict xsaves()/xrstors() to independent states

These interfaces are really only valid for features which are independently
managed and not part of the task context state for various reasons.

Tighten the checks and adjust the misleading comments.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.608492174@linutronix.de