scsi: sd: Revert "scsi: sd: Remove a local variable"

This reverts commit 84f7a9de.

Because it introduces a problem that rq->__data_len is set to the wrong
value.

before the patch:
1) nr_bytes = rq->__data_len
2) rq->__data_len = sdp->sector_size
3) scsi_init_io()
4) rq->__data_len = nr_bytes

after the patch:
1) rq->__data_len = sdp->sector_size
2) scsi_init_io()
3) rq->__data_len = rq->__data_len -> __data_len is wrong

It will cause that io can only complete one segment each time, and the io
will requeue in scsi_io_completion_action(), which will cause severe
performance degradation.

Scsi write same is removed in commit e383e16e ("scsi: sd: Remove
WRITE_SAME support") from mainline, hence this patch is only needed for
stable kernels.

Fixes: 84f7a9de ("scsi: sd: Remove a local variable")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>