sql: fix cancel leak in happy case in fingerprint_span

The recent fix introduced a possible leak of the cancel function
(namely, in the happy case when the coordinator doesn't hit an early
error, we would never call the cancellation function). We should just
defer it unconditionally and might call it earlier on a shutdown due to
an error.

Release note: None

Author: yuzefovich

pkg/sql/fingerprint_span.go

@@ -139,6 +139,7 @@ func (p *planner) fingerprintSpanFanout(
 			// the coordinator encounters an error.
 			workCh := make(chan roachpb.Span)
 			ctx, cancel := context.WithCancel(ctx)
+			defer cancel()
 
 			grp := ctxgroup.WithContext(ctx)
 			for range maxWorkerCount {