FROMLIST: drivers/iommu: Take a ref to the IOMMU driver prior to ->add_device()

To avoid accidental removal of an active IOMMU driver module, take a
reference to the driver module in 'iommu_probe_device()' immediately
prior to invoking the '->add_device()' callback and hold it until the
after the device has been removed by '->remove_device()'.

Suggested-by: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Will Deacon <will@kernel.org>
Bug: 140290589
Link: https://lore.kernel.org/lkml/20191121114918.2293-5-will@kernel.org/


Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I8f6a8aad8c41faa51df9c3a440c8b80c74e3ce8a