[SCSI] fix BUG when sum(scatterlist) > bufflen

When sending a SCSI command to a tape drive via the SCSI Generic (sg)
driver, if the command has a data transfer length more than
scatter_elem_sz (32 KB default) and not a multiple of 512, then I either
hit BUG_ON(!valid_dma_direction(direction)) in dma_unmap_sg() or else
the command never completes (depending on the LLDD).

When constructing scatterlists, the sg driver rounds up the scatterlist
element sizes to be a multiple of 512.  This can result in
sum(scatterlist lengths) > bufflen.  In this case, scsi_req_map_sg()
incorrectly sets bio->bi_size to sum(scatterlist lengths) rather than to
bufflen.  When the command completes, req_bio_endio() detects that
bio->bi_size != 0, and so it doesn't call bio_endio().  This causes the
command to be resubmitted, resulting in BUG_ON or the command never
completing.

This patch makes scsi_req_map_sg() set bio->bi_size to bufflen rather
than to sum(scatterlist lengths), which fixes the problem.
Signed-off-by: NTony Battersby <tonyb@cybernetics.com>
Acked-by: NMike Christie <michaelc@cs.wisc.edu>
Signed-off-by: NJames Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/scsi_lib.c

@@ -301,7 +301,6 @@ static int scsi_req_map_sg(struct request *rq, struct scatterlist *sgl,
 		page = sg_page(sg);
 		off = sg->offset;
 		len = sg->length;
- 		data_len += len;
 
 		while (len > 0 && data_len > 0) {
 			/*