x86, AMD IOMMU: replace memset with __GFP_ZERO for table allocation

This patch removes the memset from the data structure initialization code and
allocate the structures with the __GFP_ZERO flag.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>