x86/fpu/xstate: Sanitize handling of independent features

The copy functions for the independent features are horribly named and the
supervisor and independent part is just overengineered.

The point is that the supplied mask has either to be a subset of the
independent features or a subset of the task->fpu.xstate managed features.

Rewrite it so it checks for invalid overlaps of these areas in the caller
supplied feature mask. Rename it so it follows the new naming convention
for these operations. Mop up the function documentation.

This allows to use that function for other purposes as well.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Kan Liang <kan.liang@linux.intel.com>
Link: https://lkml.kernel.org/r/20210623121455.004880675@linutronix.de