x86 has set_bit_string() that does the exact same thing that
set_bit_area() in lib/iommu-helper.c does.

This patch exports set_bit_area() in lib/iommu-helper.c as
iommu_area_reserve(), converts GART, Calgary, and AMD IOMMU to use it.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: NJoerg Roedel <joerg.roedel@amd.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>