disk device manager: check that partitions are unmounted before uninitializing.

when uninitializing a partition or a disk (removing the partition
table), check that all partitions from that table are unmounted, as they
are about to become invalid.

Fixes #8827.

headers/private/kernel/disk_device_manager/KPartition.h

@@ -85,6 +85,7 @@ public:
 	bool ContainsPartitioningSystem() const;
 	bool IsReadOnly() const;
 	bool IsMounted() const;
+	bool IsChildMounted();
 
 	bool IsDevice() const;