iommu/iova: Reorganise some code

iommu/iova: Reorganise some code

The iova_cache_{get,put}() calls really represent top-level lifecycle
management for the whole IOVA library, so it's long been rather
confusing to have them buried right in the middle of the allocator
implementation details. Move them to a more expected position at the end
of the file, where it will then also be easier to expand them. With
this, we can also move the rcache hotplug handler (plus another stray
function) into the rcache portion of the file.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/d4753562f4faa0e6b3aeebcbf88fdb60cc22d715.1707144953.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>