gnttab: bypass IOMMU (un)mapping when a domain is (un)mapping its own grant

gnttab: bypass IOMMU (un)mapping when a domain is (un)mapping its own grant

Mappings for a domain's own pages should already be present in the
IOMMU. While installing the same mapping again is merely redundant (and
inefficient), removing the mapping when the grant mapping gets removed
is outright wrong in this case: The mapping was there before the map, so
should remain in place after unmapping.

This affects
- Arm Dom0 in the direct mapped case,
- x86 PV Dom0 in the "iommu=dom0-strict" / "dom0-iommu=strict" case,
- all x86 PV DomU-s, including driver domains.

See the code comment for why it's the original domain and not the page
owner that gets compared against.

Reported-by: Rahul Singh <Rahul.Singh@arm.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>