Infinite loop on kernel when acquiring lock for mkfs during volume creation

[flaviostutz]

I am using the exclusive-lock feature on the Ceph images. During volume creation, the kernel enters an infinite loop with a message "Locked by external element" in "dmesg -w". I have to reboot the machine then. No success trying to kill the container.

Tested on kernel 4.15 and 4.17 with Mimic.

[flaviostutz]

During my tests, during mkfs and mount, the kernel acquires a lock automatically on the image resource if running the creation and mkfs procedures outside the plugin (raw commands on rbd cli).

Maybe we could remove the custom locking capabilities from the plugin. I commented out the custom locking from the plugin and the error is not raising anymore. If my tests are successful I will provide a PR here.