iommufd: Move _iommufd_object_alloc out of driver.c

iommufd: Move _iommufd_object_alloc out of driver.c

Now, all driver structures will be allocated by the core, i.e. no longer a
need of driver calling _iommufd_object_alloc. Thus, move it back.

Before:
   text	   data	    bss	    dec	    hex	filename
   3024	    180	      0	   3204	    c84	drivers/iommu/iommufd/driver.o
   9074	    610	     64	   9748	   2614	drivers/iommu/iommufd/main.o
After:
   text	   data	    bss	    dec	    hex	filename
   2665	    164	      0	   2829	    b0d	drivers/iommu/iommufd/driver.o
   9410	    618	     64	  10092	   276c	drivers/iommu/iommufd/main.o

Link: https://patch.msgid.link/r/79e630c7b911930cf36e3c8a775a04e66c528d65.1749882255.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>