Reduce loop from running through all elements

- Just addign a 'last' statement once an extent is found. No since
looking for more after the extent is found because there is no more.
Should make plugin run a bit faster.

perl5/PVE/Storage/LunCmd/FreeNAS.pm

@@ -695,6 +695,7 @@ sub freenas_list_lu {
                         }
                         $node->{$freenas_api_variables->{'lunid'}} .= $iscsi_lunid;
                         push(@luns , $node);
+                        last;
                     }
                 }
             }