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.

Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>