m_data is used for zero clusters even though valid == 0. It really only
means that a new cluster was allocated in the image file. Rename it to
reflect this.

While at it, change it from int to bool, too.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200430133007.170335-2-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>