Now marks a partition unmounted on unmount - that allows a volume to be remounted as often as you like.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@14036 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/system/kernel/fs/vfs.cpp

@@ -5154,8 +5154,11 @@ fs_unmount(char *path, uint32 flags, bool kernel)
 	// release the file system
 	put_file_system(mount->fs);
 
-	// dereference the partition
+	// dereference the partition and mark it unmounted
 	if (partition) {
+		partition->SetVolumeID(-1);
+		partition->SetMountCookie(NULL);
+
 		if (mount->owns_file_device)
 			KDiskDeviceManager::Default()->DeleteFileDevice(partition->ID());
 		partition->Unregister();