iommu/amd: Fix NULL pointer deref on device detach

When a device group is detached from its domain, the iommu
core code calls into the iommu driver to detach each device
individually.

Before this functionality went into the iommu core code, it
was implemented in the drivers, also in the AMD IOMMU
driver as the device alias handling code.

This code is still present, as there might be aliases that
don't exist as real PCI devices (and are therefore invisible
to the iommu core code).

Unfortunatly it might happen now, that a device is unbound
multiple times from its domain, first by the alias handling
code and then by the iommu core code (or vice verca).

This ends up in the do_detach function which dereferences
the dev_data->domain pointer. When the device is already
detached, this pointer is NULL and we get a kernel oops.

Removing the alias code completly is not an option, as that
would also remove the code which handles invisible aliases.
The code could be simplified, but this is too big of a
change outside the merge window.

For now, just check the dev_data->domain pointer in
do_detach and bail out if it is NULL.

Reported-by: Andreas Hartmann <andihartmann@freenet.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>