iommu/iova: Make the rcache depot scale better

The algorithm in the original paper specifies the storage of full
magazines in the depot as an unbounded list rather than a fixed-size
array. It turns out to be pretty straightforward to do this in our
implementation with no significant loss of efficiency. This allows
the depot to scale up to the working set sizes of larger systems,
while also potentially saving some memory on smaller ones too.

Since this involves touching struct iova_magazine with the requisite
care, we may as well reinforce the comment with a proper assertion too.

Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>