scsi: sd_zbc: Ensure buffer size is aligned to SECTOR_SIZE

Reporting zones on a SCSI device sometimes fail with the following error:

[76248.516390] ata16.00: invalid transfer count 131328
[76248.523618] sd 15:0:0:0: [sda] REPORT ZONES start lba 536870912 failed

The error (from drivers/ata/libata-scsi.c:ata_scsi_zbc_in_xlat()) indicates
that buffer size is not aligned to SECTOR_SIZE.

This happens when the __vmalloc() failed. Consider we are reporting 4096
zones, then we will have "bufsize = roundup((4096 + 1) * 64,
SECTOR_SIZE)" = (513 * 512) = 262656. Then, __vmalloc() failure halves
the bufsize to 131328, which is no longer aligned to SECTOR_SIZE.

Use rounddown() to ensure the size is always aligned to SECTOR_SIZE and fix
the comment as well.

Link: https://lore.kernel.org/r/20210906140642.2267569-1-naohiro.aota@wdc.com
Fixes: 23a50861

 ("scsi: sd_zbc: Cleanup sd_zbc_alloc_report_buffer()")
Cc: stable@vger.kernel.org # 5.5+
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>