[mle] suppress parent response when >= 32 active routers (#5156)

This commit introduces one extra parent response rule:
REED doesn't respond parent request if there are already
`kMaxRouters` active routers in the Thread network as it's
for sure that Leader will reject its address solicit.

src/core/thread/mle_router.cpp

@@ -1612,7 +1612,7 @@ void MleRouter::HandleParentRequest(const Message &aMessage, const Ip6::MessageI
 
     VerifyOrExit(IsRouterEligible(), error = OT_ERROR_INVALID_STATE);
 
-    // A Router MUST NOT send an MLE Parent Response if:
+    // A Router/REED MUST NOT send an MLE Parent Response if:
 
     // 0. It is detached or attempting to another partition
     VerifyOrExit(!IsDetached() && !IsAttaching(), error = OT_ERROR_DROP);
@@ -1635,6 +1635,10 @@ void MleRouter::HandleParentRequest(const Message &aMessage, const Ip6::MessageI
                      (leader->GetCost() + GetLinkCost(leader->GetNextHop()) < kMaxRouteCost),
                  error = OT_ERROR_DROP);
 
+    // 4. It is a REED and there are already `kMaxRouters` active routers in
+    // the network (because Leader would reject any further address solicit).
+    // ==> Verified below when checking the scan mask.
+
     aMessageInfo.GetPeerAddr().ToExtAddress(macAddr);
 
     // Version
@@ -1652,6 +1656,7 @@ void MleRouter::HandleParentRequest(const Message &aMessage, const Ip6::MessageI
 
     case kRoleChild:
         VerifyOrExit(ScanMaskTlv::IsEndDeviceFlagSet(scanMask), OT_NOOP);
+        VerifyOrExit(mRouterTable.GetActiveRouterCount() < kMaxRouters, error = OT_ERROR_DROP);
         break;
 
     case kRoleRouter: