x86: iommu_sac_force can become static

The iommu_sac_force variable is needlessly defined global,
and this patch makes it static. Additionally, this variable
needs not be explicitly initialized.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>