md-cluster: add the error check if failed to get dlm lock

In complicated cluster environment, it is possible that the
dlm lock couldn't be get/convert on purpose, the related err
info is added for better debug potential issue.

For lockres_free, if the lock is blocking by a lock request or
conversion request, then dlm_unlock just put it back to grant
queue, so need to ensure the lock is free finally.

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>