* mount.cc (class fs_info_cache): New class to cache filesystem

	* mount.cc (class fs_info_cache): New class to cache filesystem
	information.
	(fs_info::update): Check FileFsVolumeInformation against filesystem
	cache and use it, if filesystem is already available.  Add filesystem
	to cache, if not.  Only request FileFsObjectIdInformation if
	FILE_SUPPORTS_OBJECT_IDS is set in filesystem flags.
	* ntdll.h (struct _FILE_FS_VOLUME_INFORMATION): Add pragma pack so the
	structure size is matching the OS expectations.  Add __dummy member
	used in filesystem cache.