sstable: fix invariant check for sstable size estimation

Now that we have ways to determine whether parallelism is enabled, we
should check it, before making the invariant check for the no parallelism
case.

sstable/writer.go

@@ -1813,11 +1813,10 @@ func (w *Writer) Close() (err error) {
 // EstimatedSize returns the estimated size of the sstable being written if a
 // call to Finish() was made without adding additional keys.
 func (w *Writer) EstimatedSize() uint64 {
-	if invariants.Enabled {
-		// Assuming this is the parallelism disabled case. Need, #1581 to merge
-		// before we can make that check. The w.meta.Size should only be
-		// accessed from the writeQueue goroutine if parallelism is enabled,
-		// but since it isn't we break that invariant here.
+	if invariants.Enabled && !w.coordination.parallelismEnabled {
+		// The w.meta.Size should only be accessed from the writeQueue goroutine
+		// if parallelism is enabled, but since it isn't we break that invariant
+		// here.
 		if w.coordination.sizeEstimate.size() != w.meta.Size {
 			panic("sstable size estimation sans parallelism is incorrect")
 		}