iova: Remove iovad->rcaches check in iova_rcache_get()

The iovad->rcaches check in iova_rcache_get() is pretty much useless
without the same check in iova_rcache_insert().

Instead of adding this symmetric check to fastpath iova_rcache_insert(),
drop the check in iova_rcache_get() in favour of making the IOVA domain
rcache init more robust to failure in future.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/1662557681-145906-4-git-send-email-john.garry@huawei.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>