iommu: Revisit iommu_insert_resv_region() implementation

Current implementation is recursive and in case of allocation
failure the existing @regions list is altered. A non recursive
version looks better for maintainability and simplifies the
error handling. We use a separate stack for overlapping segment
merging. The elements are sorted by start address and then by
type, if their start address match.

Note this new implementation may change the region order of
appearance in /sys/kernel/iommu_groups/<n>/reserved_regions
files but this order has never been documented, see
commit bc7d12b9

 ("iommu: Implement reserved_regions
iommu-group sysfs file").

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>