listing: Fix result when prefix is an object with a slash (#10267)

In a non recursive mode, issuing a list request where prefix
is an existing object with a slash and delimiter is a slash will
return entries in the object directory (data dir IDs)

```
$ aws s3api --profile minioadmin --endpoint-url http://localhost:9000 \
        list-objects-v2 --bucket testbucket --prefix code_of_conduct.md/ --delimiter '/'
{
    "CommonPrefixes": [
        {
            "Prefix":
"code_of_conduct.md/ec750fe0-ea7e-4b87-bbec-1e32407e5e47/"
        }
    ]
}
```

This commit adds a fast exit track in Walk() in this specific case.

cmd/xl-storage.go

@@ -949,6 +949,14 @@ func (s *xlStorage) Walk(volume, dirPath, marker string, recursive bool, endWalk
 		return nil, err
 	}
 
+	// Fast exit track to check if we are listing an object with
+	// a trailing slash, this will avoid to list the object content.
+	if HasSuffix(dirPath, SlashSeparator) {
+		if st, err := os.Stat(pathJoin(volumeDir, dirPath, xlStorageFormatFile)); err == nil && st.Mode().IsRegular() {
+			return nil, errFileNotFound
+		}
+	}
+
 	// buffer channel matches the S3 ListObjects implementation
 	ch = make(chan FileInfo, maxObjectList)
 	go func() {