iommu: Fix a leak in iommu_insert_resv_region

In case we expand an existing region, we unlink
this latter and insert the larger one. In
that case we should free the original region after
the insertion. Also we can immediately return.

Fixes: 6c65fb31 ("iommu: iommu_get_group_resv_regions")

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