fix batch snowball to close channel after listing finishes (#19316)

panic seen due to premature closing of slow channel while listing is still sending or
list has already closed on the sender's side:
```
panic: close of closed channel

goroutine 13666 [running]:
github.com/minio/minio/internal/ioutil.SafeClose[...](0x101ff51e4?)
	/Users/kp/code/src/github.com/minio/minio/internal/ioutil/ioutil.go:425 +0x24
github.com/minio/minio/cmd.(*erasureServerPools).Walk.func1()
	/Users/kp/code/src/github.com/minio/minio/cmd/erasure-server-pool.go:2142 +0x170
created by github.com/minio/minio/cmd.(*erasureServerPools).Walk in goroutine 1189
	/Users/kp/code/src/github.com/minio/minio/cmd/erasure-server-pool.go:1985 +0x228
```

cmd/batch-handlers.go

@@ -1064,8 +1064,6 @@ func (r *BatchJobReplicateV1) Start(ctx context.Context, api ObjectLayer, job Ba
 
 	if !*r.Source.Snowball.Disable && r.Source.Type.isMinio() && r.Target.Type.isMinio() {
 		go func() {
-			defer xioutil.SafeClose(slowCh)
-
 			// Snowball currently needs the high level minio-go Client, not the Core one
 			cl, err := miniogo.New(u.Host, &miniogo.Options{
 				Creds:        credentials.NewStaticV4(cred.AccessKey, cred.SecretKey, cred.SessionToken),
@@ -1081,31 +1079,8 @@ func (r *BatchJobReplicateV1) Start(ctx context.Context, api ObjectLayer, job Ba
 			// Already validated before arriving here
 			smallerThan, _ := humanize.ParseBytes(*r.Source.Snowball.SmallerThan)
 
-			var (
-				obj   = ObjectInfo{}
-				batch = make([]ObjectInfo, 0, *r.Source.Snowball.Batch)
-				valid = true
-			)
-
-			for valid {
-				obj, valid = <-walkCh
-
-				if !valid {
-					goto write
-				}
-
-				if obj.DeleteMarker || !obj.VersionPurgeStatus.Empty() || obj.Size >= int64(smallerThan) {
-					slowCh <- obj
-					continue
-				}
-
-				batch = append(batch, obj)
-
-				if len(batch) < *r.Source.Snowball.Batch {
-					continue
-				}
-
-			write:
+			batch := make([]ObjectInfo, 0, *r.Source.Snowball.Batch)
+			writeFn := func(batch []ObjectInfo) {
 				if len(batch) > 0 {
 					if err := r.writeAsArchive(ctx, api, cl, batch); err != nil {
 						logger.LogIf(ctx, err)
@@ -1118,9 +1093,24 @@ func (r *BatchJobReplicateV1) Start(ctx context.Context, api ObjectLayer, job Ba
 						// persist in-memory state to disk after every 10secs.
 						logger.LogIf(ctx, ri.updateAfter(ctx, api, 10*time.Second, job))
 					}
-					batch = batch[:0]
 				}
 			}
+			for obj := range walkCh {
+				if obj.DeleteMarker || !obj.VersionPurgeStatus.Empty() || obj.Size >= int64(smallerThan) {
+					slowCh <- obj
+					continue
+				}
+
+				batch = append(batch, obj)
+
+				if len(batch) < *r.Source.Snowball.Batch {
+					continue
+				}
+				writeFn(batch)
+				batch = batch[:0]
+			}
+			writeFn(batch)
+			xioutil.SafeClose(slowCh)
 		}()
 	} else {
 		slowCh = walkCh