iommu: Factor iommu_iotlb_gather_is_disjoint() out

Refactor iommu_iotlb_gather_add_page() and factor out the logic that
detects whether IOTLB gather range and a new range are disjoint. To be
used by the next patch that implements different gathering logic for
AMD.

Note that updating gather->pgsize unconditionally does not affect
correctness as the function had (and has) an invariant, in which
gather->pgsize always represents the flushing granularity of its range.
Arguably, “size" should never be zero, but lets assume for the matter of
discussion that it might.

If "size" equals to "gather->pgsize", then the assignment in question
has no impact.

Otherwise, if "size" is non-zero, then iommu_iotlb_sync() would
initialize the size and range (see iommu_iotlb_gather_init()), and the
invariant is kept.

Otherwise, "size" is zero, and "gather" already holds a range, so
gather->pgsize is non-zero and (gather->pgsize && gather->pgsize !=
size) is true. Therefore, again, iommu_iotlb_sync() would be called and
initialize the size.

Cc: Joerg Roedel <joro@8bytes.org>
Cc: Jiajun Cao <caojiajun@vmware.com>
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Cc: iommu@lists.linux-foundation.org
Cc: linux-kernel@vger.kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
Link: https://lore.kernel.org/r/20210723093209.714328-5-namit@vmware.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

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