Add an mmu_notifier for protected VMs. The callback function is
triggered when the mm is torn down, and will attempt to convert all
protected vCPUs to non-protected. This allows the mm teardown to use
the destroy page UVC instead of export.

Also make KVM select CONFIG_MMU_NOTIFIER, needed to use mmu_notifiers.
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Link: https://lore.kernel.org/r/20220628135619.32410-10-imbrenda@linux.ibm.com
Message-Id: <20220628135619.32410-10-imbrenda@linux.ibm.com>
[frankja@linux.ibm.com: Conflict resolution for mmu_notifier.h include
and struct kvm_s390_pv]
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>