For preparation of LU Dedicated buffer mode support on WriteBooster
feature, "index" parameter shall be added and allowed to be specified by
callers.

Link: https://lore.kernel.org/r/20200508080115.24233-6-stanley.chu@mediatek.comReviewed-by: Bean Huo <beanhuo@micron.com>
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Can Guo <cang@codeaurora.org>
Reviewed-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>