iommu/vt-d: Set ISA bridge reserved region as relaxable

Commit d850c2ee ("iommu/vt-d: Expose ISA direct mapping region via
iommu_get_resv_regions") created a direct-mapped reserved memory region
in order to replace the static identity mapping of the ISA address
space, where the latter was then removed in commit df4f3c60
("iommu/vt-d: Remove static identity map code").  According to the
history of this code and the Kconfig option surrounding it, this direct
mapping exists for the benefit of legacy ISA drivers that are not
compatible with the DMA API.

In conjuntion with commit 9b77e5c7 ("vfio/type1: check dma map
request is within a valid iova range") this change introduced a
regression where the vfio IOMMU backend enforces reserved memory regions
per IOMMU group, preventing userspace from creating IOMMU mappings
conflicting with prescribed reserved regions.  A necessary prerequisite
for the vfio change was the introduction of "relaxable" direct mappings
introduced by commit adfd3738 ("iommu: Introduce
IOMMU_RESV_DIRECT_RELAXABLE reserved memory regions").  These relaxable
direct mappings provide the same identity mapping support in the default
domain, but also indicate that the reservation is software imposed and
may be relaxed under some conditions, such as device assignment.

Convert the ISA bridge direct-mapped reserved region to relaxable to
reflect that the restriction is self imposed and need not be enforced
by drivers such as vfio.

Fixes: 1c5c59fb

 ("iommu/vt-d: Differentiate relaxable and non relaxable RMRRs")
Cc: stable@vger.kernel.org # v5.3+
Link: https://lore.kernel.org/linux-iommu/20191211082304.2d4fab45@x1.home
Reported-by: cprt <cprt@protonmail.com>
Tested-by: cprt <cprt@protonmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>