x86: silence section mismatch warning in smpboot_64.c

Silence the following warning:
WARNING: o-x86_64/arch/x86/kernel/built-in.o(.text+0x17cd3): Section mismatch in reference from the function remove_cpu_from_maps() to the variable .cpuinit.data:cpu_initialized

remove_cpu:maps() had a single user: __cpu_disable() so
mark it static and annotate it with __ref to silence the
warning from modpost.

_cpu_disable() has a single user in kernel/cpu.c:
 => take_cpu_down()
    which again has a single user in the following call:
    => __stop_machine_run(take_cpu_down, &tcd_param, cpu);
Here a kthread is created.

So maybe the warning is correct and the right fix is to
remove the __cpuinitdata annotation of cpu_initialized?

Note: The analysis were disturbed by the fact that we had a variable
      with the same name in cpu/common.c - but this is 32 bit only]
Note: Should smpboot_64 use cpu_clear()?

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>