rbd: support for exclusive-lock feature

Add basic support for RBD_FEATURE_EXCLUSIVE_LOCK feature.  Maintenance
operations (resize, snapshot create, etc) are offloaded to librbd via
returning -EOPNOTSUPP - librbd should request the lock and execute the
operation.

Signed-off-by: Ilya Dryomov <[email protected]>
Reviewed-by: Mike Christie <[email protected]>
Tested-by: Mike Christie <[email protected]>

Author: idryomov