iommu/iova: Tidy up iova_cache_get() failure

iommu/iova: Tidy up iova_cache_get() failure

Failure handling in iova_cache_get() is a little messy, and we'd like
to add some more to it, so let's tidy up a bit first. By leaving the
hotplug handler until last we can take advantage of kmem_cache_destroy()
being NULL-safe to have a single cleanup label. We can also improve the
error reporting, noting that kmem_cache_create() already screams if it
fails, so that one is redundant.

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/ae4a3bda2d6a9b738221553c838d30473bd624e7.1707144953.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>