iommufd: Fix missing update of domains_itree after splitting iopt_area

In iopt_area_split(), if the original iopt_area has filled a domain and is
linked to domains_itree, pages_nodes have to be properly
reinserted. Otherwise the domains_itree becomes corrupted and we will UAF.

Fixes: 51fe6141

 ("iommufd: Data structure to provide IOVA to PFN mapping")
Link: https://lore.kernel.org/r/20231027162941.2864615-2-den@valinux.co.jp
Cc: stable@vger.kernel.org
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>