rbd-target-api: get image's exclusive lock owner and break it

[lxbsz]

Once the client logged in and IOs are done, one of the iscsi gateway nodes will hold the exclusive lock and be the lock owner in tcmu-runner.

When we try rollback the snap shot of the image, it will become readonly and failed even DELETE the LUN from the client host.

This patch will just get the lock owner and break it when doing the LUN's DELETE opration.

Signed-off-by: Zhang Zhuoyu [email protected] Signed-off-by: Xiubo Li [email protected]

[zhuozh]

Creating snapshot fail like this: