storage: defer putBuffer release in all cases

This commit switches the remainder of the calls to putBuffer.release to be
deferred, instead of being manually called at the end of their function. The
comments mentioning that the defer was "measurably slower" were introduced in
4444618, which was before Go 1.14 optimized the performance of defer. Most of
these, including the more performance sensitive calls, were already switched
over to use defer in fbe8852.

Epic: None
Release note: None

pkg/storage/mvcc.go

@@ -2547,6 +2547,7 @@ func MVCCMerge(
 	metaKey := MakeMVCCMetadataKey(key)
 
 	buf := newPutBuffer()
+	defer buf.release()
 
 	// Every type flows through here, so we can't use the typed getters.
 	rawBytes := value.RawBytes
@@ -2566,7 +2567,6 @@ func MVCCMerge(
 				key, int64(len(rawBytes)), timestamp.WallTime))
 		}
 	}
-	buf.release()
 	return err
 }
 
@@ -4342,6 +4342,7 @@ func MVCCResolveWriteIntent(
 		return false, 0, nil, err
 	}
 	iterAndBuf := GetBufUsingIter(iter)
+	defer iterAndBuf.Cleanup()
 	iterAndBuf.iter.SeekIntentGE(intent.Key, intent.Txn.ID)
 	// Production code will use a buffered writer, which makes the numBytes
 	// calculation accurate. Note that an inaccurate numBytes (e.g. 0 in the
@@ -4350,8 +4351,6 @@ func MVCCResolveWriteIntent(
 	beforeBytes := rw.BufferedSize()
 	ok, err = mvccResolveWriteIntent(ctx, rw, iterAndBuf.iter, ms, intent, iterAndBuf.buf)
 	numBytes = int64(rw.BufferedSize() - beforeBytes)
-	// Using defer would be more convenient, but it is measurably slower.
-	iterAndBuf.Cleanup()
 	return ok, numBytes, nil, err
 }
 
@@ -5164,6 +5163,7 @@ func MVCCResolveWriteIntentRange(
 	if err != nil {
 		return 0, 0, nil, 0, err
 	}
+	defer engineIter.Close()
 	var mvccIter MVCCIterator
 	iterOpts := IterOptions{
 		KeyTypes:   IterKeyTypePointsAndRanges,
@@ -5181,10 +5181,7 @@ func MVCCResolveWriteIntentRange(
 		mvccIter = newPebbleIteratorByCloning(engineIter.CloneContext(), iterOpts, StandardDurability)
 	}
 	iterAndBuf := GetBufUsingIter(mvccIter)
-	defer func() {
-		engineIter.Close()
-		iterAndBuf.Cleanup()
-	}()
+	defer iterAndBuf.Cleanup()
 	putBuf := iterAndBuf.buf
 	sepIter := &separatedIntentAndVersionIter{
 		engineIter: engineIter,