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.

src/system/kernel/disk_device_manager/KPartition.cpp

@@ -559,6 +559,20 @@ KPartition::IsMounted() const
 }
 
 
+bool
+KPartition::IsChildMounted()
+{
+	struct IsMountedVisitor : KPartitionVisitor {
+		virtual bool VisitPre(KPartition* partition)
+		{
+			return partition->IsMounted();
+		}
+	} checkVisitor;
+
+	return VisitEachDescendant(&checkVisitor) != NULL;
+}
+
+
 bool
 KPartition::IsDevice() const
 {