Another thing. Please check for containers referencing the filesystems in question. When you have _netdev filesystems, it is possible that the "mount --shared" characteristics of containers (different mount namescapes) are holding references to the filesystem in question.
A good test is to test the filesystem being mounted by /etc/fstab AND manually. If mounted with _netdev, by /etc/fstab, then the containers will have reference to the same filesystems as a shared mount. This will cause a shutdown problem if you don't remove the container reference as well.
Another thing. Please check for containers referencing the filesystems in question. When you have _netdev filesystems, it is possible that the "mount --shared" characteristics of containers (different mount namescapes) are holding references to the filesystem in question.
A good test is to test the filesystem being mounted by /etc/fstab AND manually. If mounted with _netdev, by /etc/fstab, then the containers will have reference to the same filesystems as a shared mount. This will cause a shutdown problem if you don't remove the container reference as well.
Please do test this as well.