iommu: dmar: Provide helper to copy shared irte fields

Instead of open coding, provide a helper function to copy the shared
irte fields.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: jiang.liu@linux.intel.com
Cc: iommu@lists.linux-foundation.org
Cc: joro@8bytes.org
Cc: dwmw2@infradead.org
Link: http://lkml.kernel.org/r/1433827237-3382-4-git-send-email-feng.wu@intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>