LU-14455 mdt: fix DoM lock prolong logic

LU-14455 mdt: fix DoM lock prolong logic

- don't stop at the first found lock if it is not PW or EX lock
- add LCK_GROUP lock as valid mode of lock to check

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: If947f8565008953cc34146b6f0ac1e0f0a038bb5
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-on: https://review.whamcloud.com/41857
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>