scsi: ufs: Use data structure size in pointer arithmetic

Use the structure size in pointer arithmetic instead of an opaque 32
bytes for the over-allocation of descriptors.

Signed-off-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>