iommu/amd: Implement mmu_notifier_release call-back

Since mmu_notifier call-backs can sleep (because they use
SRCU now) we can use them to tear down PASID mappings. This
allows us to finally remove the hack to use the task_exit
notifier from oprofile to get notified when a process dies.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Jay Cornwall <Jay.Cornwall@amd.com>