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: NBean Huo <beanhuo@micron.com>
Reviewed-by: NAvri Altman <avri.altman@wdc.com>
Reviewed-by: NCan Guo <cang@codeaurora.org>
Reviewed-by: NAsutosh Das <asutoshd@codeaurora.org>
Signed-off-by: NStanley Chu <stanley.chu@mediatek.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>