Validate block header gas limit

* Add block header gas limit validation in ValidateBlock function,
  see eq 39 and 45 in yellow paper. Before it was calculated _for_
  the block instead of validated.
* Use the block header gas limit when setting the gas pool instead
  of calculating the value for the block.

core/block_processor.go

@@ -62,7 +62,7 @@ func NewBlockProcessor(db ethutil.Database, txpool *TxPool, chainManager *ChainM
 
 func (sm *BlockProcessor) TransitionState(statedb *state.StateDB, parent, block *types.Block, transientProcess bool) (receipts types.Receipts, err error) {
 	coinbase := statedb.GetOrNewStateObject(block.Header().Coinbase)
-	coinbase.SetGasPool(CalcGasLimit(parent, block))
+	coinbase.SetGasPool(block.Header().GasLimit)
 
 	// Process the transactions on to parent state
 	receipts, _, _, _, err = sm.ApplyTransactions(coinbase, statedb, block, block.Transactions(), transientProcess)
@@ -247,6 +247,11 @@ func (sm *BlockProcessor) ValidateBlock(block, parent *types.Block) error {
 		return fmt.Errorf("Difficulty check failed for block %v, %v", block.Header().Difficulty, expd)
 	}
 
+	expl := CalcGasLimit(parent, block)
+	if expl.Cmp(block.Header().GasLimit) != 0 {
+		return fmt.Errorf("GasLimit check failed for block %v, %v", block.Header().GasLimit, expl)
+	}
+
 	if block.Time() < parent.Time() {
 		return ValidationError("Block timestamp not after prev block (%v - %v)", block.Header().Time, parent.Header().Time)
 	}