x86/fpu: Mark init_fpstate __ro_after_init

Nothing has to write into that state after init.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121456.992342060@linutronix.de