add lu_op_free(), use in luop.c

Add to luop.h, it will be used in db_hlr.c in an upcoming patch.

Change-Id: Ib44d9062edc957d2e0710b7e485604f97e4d5612
diff --git a/src/luop.h b/src/luop.h
index ab1bc24..053a025 100644
--- a/src/luop.h
+++ b/src/luop.h
@@ -79,3 +79,5 @@
 void lu_op_tx_cancel_old(struct lu_operation *luop);
 void lu_op_tx_insert_subscr_data(struct lu_operation *luop);
 void lu_op_tx_del_subscr_data(struct lu_operation *luop);
+
+void lu_op_free(struct lu_operation *luop);