x86/fpu: Move init_xstate_size() to __init section

WARNING: vmlinux.o(.text.unlikely+0x1c05): Section mismatch in
         reference from the function init_xstate_size() to the
         function .init.text:get_xsave_size()

WARNING: vmlinux.o(.text.unlikely+0x1c19): Section mismatch in
         reference from the function init_xstate_size() to the
         function .init.text:get_xsaves_size()

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Link: https://lkml.kernel.org/r/20190108130225.5066-2-sergey.senozhatsky@gmail.com