scsi: core: Delete unnecessary buffer allocation for every loop iteration

No need to allocate buffer every loop iteration. Allocate buffer once and
reuse it.

Link: https://lore.kernel.org/r/20200717090921.29243-2-yebin10@huawei.com


Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>