network/tc: allow to configure class or qdisc under foreign one

Some qdiscs (e.g. tbf) implicitly create class(es) on create.
Previously, we could not create any child qdisc under the class, as the
implicit class is tagged as foreign.

src/network/tc/qdisc.c

@@ -276,9 +276,6 @@ int link_find_qdisc(Link *link, uint32_t handle, uint32_t parent, const char *ki
                 if (qdisc->parent != parent)
                         continue;
 
-                if (qdisc->source == NETWORK_CONFIG_SOURCE_FOREIGN)
-                        continue;
-
                 if (!qdisc_exists(qdisc))
                         continue;
 

src/network/tc/tclass.c

@@ -223,9 +223,6 @@ int link_find_tclass(Link *link, uint32_t classid, TClass **ret) {
                 if (tclass->classid != classid)
                         continue;
 
-                if (tclass->source == NETWORK_CONFIG_SOURCE_FOREIGN)
-                        continue;
-
                 if (!tclass_exists(tclass))
                         continue;