b2: don't include the bucket name in public link file prefixes

Including the bucket name as part of the `fileNamePrefix` passed to
`b2_get_download_authorization` results in a link valid for objects that
have the bucket name as part of the object path; e.g.,

    rclone link :b2:some-bucket/some-file

would result in a public link valid for the object
`some-bucket/some-file` in the `some-bucket` bucket (in rclone-remote
parlance, `:b2:some-bucket/some-bucket/some-file`). This will almost
certainly result in a broken link.

The B2 docs don't explicitly specify this behavior, but the example
given for `fileNamePrefix` provides some clarification.

See https://www.backblaze.com/b2/docs/b2_get_download_authorization.html.

backend/b2/b2.go

@@ -1353,7 +1353,7 @@ func (f *Fs) getDownloadAuthorization(ctx context.Context, bucket, remote string
 	}
 	var request = api.GetDownloadAuthorizationRequest{
 		BucketID:               bucketID,
-		FileNamePrefix:         f.opt.Enc.FromStandardPath(path.Join(f.root, remote)),
+		FileNamePrefix:         f.opt.Enc.FromStandardPath(path.Join(f.rootDirectory, remote)),
 		ValidDurationInSeconds: validDurationInSeconds,
 	}
 	var response api.GetDownloadAuthorizationResponse