tasks/mds_client_recovery: file lock test

check that file lock doesn't get lost after an MDS restart

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #181 from ceph/wip-client-flock

tasks/mds_client_recovery: file lock test

Reviewed-by: Greg Farnum <greg@inktank.com>