iommu: Rename ops->domain_alloc_user() to domain_alloc_paging_flags()

iommu: Rename ops->domain_alloc_user() to domain_alloc_paging_flags()

Now that the main domain allocating path is calling this function it
doesn't make sense to leave it named _user. Change the name to
alloc_paging_flags() to mirror the new iommu_paging_domain_alloc_flags()
function.

A driver should implement only one of ops->domain_alloc_paging() or
ops->domain_alloc_paging_flags(). The former is a simpler interface with
less boiler plate that the majority of drivers use. The latter is for
drivers with a greater feature set (PASID, multiple page table support,
advanced iommufd support, nesting, etc). Additional patches will be needed
to achieve this.

Link: https://patch.msgid.link/r/2-v1-c252ebdeb57b+329-iommu_paging_flags_jgg@nvidia.com
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>