iommufd: Move _iommufd_object_alloc helper to a sharable file

iommufd: Move _iommufd_object_alloc helper to a sharable file

The following patch will add a new vIOMMU allocator that will require this
_iommufd_object_alloc to be sharable with IOMMU drivers (and iommufd too).

Add a new driver.c file that will be built with CONFIG_IOMMUFD_DRIVER_CORE
selected by CONFIG_IOMMUFD, and put the CONFIG_DRIVER under that remaining
to be selectable for drivers to build the existing iova_bitmap.c file.

Link: https://patch.msgid.link/r/2f4f6e116dc49ffb67ff6c5e8a7a8e789ab9e98e.1730836219.git.nicolinc@nvidia.com
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>