x86: Move alloc_desk_mask variables inside ifdef

They are only useful with CONFIG_CPUMASK_OFFSTACK
Avoids hundreds of warnings with a gcc 4.6 -Wall build.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>