x86: fix crash on cpu hotplug on pat-incapable machines

pat_disable() is __init, which means it goes away after booting is complete.
Unfortunately it is used by the hotplug code if the machine is not
pat-capable, causing a crash.

Fix by marking pat_disable() as __cpuinit.

Signed-off-by: Avi Kivity <avi@qumranet.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>