iommu: Fix bus notifier breakage

iommu_bus_init() registers a bus notifier on the given bus by using
a statically defined notifier block:

  static struct notifier_block iommu_bus_nb = {
          .notifier_call = iommu_bus_notifier,
  };

This same notifier block is used for all busses. This causes a
problem for notifiers registered after iommu has registered this
callback on multiple busses. The problem is that a subsequent
notifier being registered on a bus which has this iommu notifier
will also get linked in to the notifier list of all other busses
which have this iommu notifier.

This patch fixes this by allocating the notifier_block at runtime.
Some error checking is also added to catch any allocation failure
or notifier registration error.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>