ocfs2/dlm: Migrate lockres with no locks if it has a reference

o2dlm was not migrating resources with zero locks because it assumed that that
resource would get purged by dlm_thread. However, some usage patterns involve
creating and dropping locks at a high rate leading to the migrate thread seeing
zero locks but the purge thread seeing an active reference. When this happens,
the dlm_thread cannot purge the resource and the migrate thread sees no reason
to migrate that resource. The spell is broken when the migrate thread catches
the resource with a lock.

The fix is to make the migrate thread also consider the reference map.

This usage pattern can be triggered by userspace on userdlm locks and flocks.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>