We only need to allocate mapping if there is an IOMMU domain.

Otherwise, when the mappings are released, the assumption that
an IOMMU domain is there will crash and burn.

CC: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[ohad: revise commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>