x86/fpu/regset: Convert to fpstate

Convert regset related code to the new register storage mechanism in
preparation for dynamically sized buffers.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211013145322.555239736@linutronix.de