iommu/iova: Manage the depot list size

iommu/iova: Manage the depot list size

Automatically scaling the depot up to suit the peak capacity of a
workload is all well and good, but it would be nice to have a way to
scale it back down again if the workload changes. To that end, add
backround reclaim that will gradually free surplus magazines if the
depot size remains above a reasonable threshold for long enough.

Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/03170665c56d89c6ce6081246b47f68d4e483308.1694535580.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>