staging: lustre: ldlm: tidy list walking in ldlm_flock()
Use list_for_each_entry variants to
avoid the explicit list_entry() calls.
This allows us to use list_for_each_entry_safe_from()
instread of adding a local list-walking macro.
Also improve some comments so that it is more obvious
that the locks are sorted per-owner and that we need
to find the insertion point.