x86/fpu/xstate: Mark all init only functions __init

No point to keep them around after boot.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011539.017919252@linutronix.de