UPSTREAM: scsi: ufs: Customize flush threshold for WriteBooster

Allow flush threshold for WriteBooster to be customizable by vendors. To
achieve this, make the value a variable in struct ufs_hba_variant_params.

Also introduce UFS_WB_BUF_REMAIN_PERCENT() macro to provide a more flexible
way to specify WriteBooster available buffer values.

Link: https://lore.kernel.org/r/20200509093716.21010-4-stanley.chu@mediatek.com


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>

Bug: 151050916
(cherry picked from commit d14734ae3ae79d3b0286a2431782054e5066da8f)
Change-Id: I6ff47b50d2f83c66ac541c8a1b6206a688662037
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>