txpool: apply miner's gasceil to txpool

Even the node did not enable the --miner option, the `Eth.Miner.GasCeil`
should also apply to txpool. Otherwise, there could be accmulated pending
transactions in txpool.
Take an example:
- Currently, BSC testnet's block gaslimit is 70M, as there is extra 20M reserved for
  SystemTxs, the maximum acceptable GasLimit for transaction is 50M.
- TxPool recevied a transaction with GasLimit 60M, it would be accepted in TxPool but it
  will never be accepted by validators. Such kinds of transactions will be kept in txpool
  and gradually make the txpool full

cmd/geth/main.go

@@ -443,22 +443,23 @@ func startNode(ctx *cli.Context, stack *node.Node, backend ethapi.Backend, isCon
 	}
 
 	// Start auxiliary services if enabled
+	ethBackend, ok := backend.(*eth.EthAPIBackend)
+	gasCeil := ethBackend.Miner().GasCeil()
+	if gasCeil > params.SystemTxsGas {
+		ethBackend.TxPool().SetMaxGas(gasCeil - params.SystemTxsGas)
+	}
 	if ctx.Bool(utils.MiningEnabledFlag.Name) {
 		// Mining only makes sense if a full Ethereum node is running
 		if ctx.String(utils.SyncModeFlag.Name) == "light" {
 			utils.Fatalf("Light clients do not support mining")
 		}
-		ethBackend, ok := backend.(*eth.EthAPIBackend)
+
 		if !ok {
 			utils.Fatalf("Ethereum service not running")
 		}
 		// Set the gas price to the limits from the CLI and start mining
 		gasprice := flags.GlobalBig(ctx, utils.MinerGasPriceFlag.Name)
 		ethBackend.TxPool().SetGasTip(gasprice)
-		gasCeil := ethBackend.Miner().GasCeil()
-		if gasCeil > params.SystemTxsGas {
-			ethBackend.TxPool().SetMaxGas(gasCeil - params.SystemTxsGas)
-		}
 		if err := ethBackend.StartMining(); err != nil {
 			utils.Fatalf("Failed to start mining: %v", err)
 		}

eth/api_miner.go

@@ -73,7 +73,7 @@ func (api *MinerAPI) SetGasPrice(gasPrice hexutil.Big) bool {
 // SetGasLimit sets the gaslimit to target towards during mining.
 func (api *MinerAPI) SetGasLimit(gasLimit hexutil.Uint64) bool {
 	api.e.Miner().SetGasCeil(uint64(gasLimit))
-	if api.e.Miner().Mining() && uint64(gasLimit) > params.SystemTxsGas {
+	if uint64(gasLimit) > params.SystemTxsGas {
 		api.e.TxPool().SetMaxGas(uint64(gasLimit) - params.SystemTxsGas)
 	}
 	return true