core: fix state flushing for catalyst mode (#26319)

The gcproc field tracks the amount of time spent processing blocks,
and is used to trigger a state flush to disk when a certain threshold is
reached. After the merge, single block insertion by CL is the most
common source of block processing time, but this time was not added
into gcproc.

core/blockchain.go

@@ -1764,6 +1764,10 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals, setHead bool)
 		stats.report(chain, it.index, dirty, setHead)
 
 		if !setHead {
+			// After merge we expect few side chains. Simply count
+			// all blocks the CL gives us for GC processing time
+			bc.gcproc += proctime
+
 			return it.index, nil // Direct block insertion of a single block
 		}
 		switch status {