b2: for a bucket limited application key check the bucket name

Before this fix rclone would just use the authorised bucket regardless
of what bucket you put on the command line.

This uses the new `bucketName` response in the API and checks that the
user is using the correct bucket name to avoid accidents.

Fixes #2839

backend/b2/b2.go

@@ -368,6 +368,13 @@ func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
 	}
 	// If this is a key limited to a single bucket, it must exist already
 	if f.bucket != "" && f.info.Allowed.BucketID != "" {
+		allowedBucket := f.info.Allowed.BucketName
+		if allowedBucket == "" {
+			return nil, errors.New("bucket that application key is restricted to no longer exists")
+		}
+		if allowedBucket != f.bucket {
+			return nil, errors.Errorf("you must use bucket %q with this application key", allowedBucket)
+		}
 		f.markBucketOK()
 		f.setBucketID(f.info.Allowed.BucketID)
 	}