[SCSI] libsrp: fix bug in ADDITIONAL CDB LENGTH interpretation

Fix a bug in the interpretation of the ADDITIONAL CDB LENGTH (add_cdb_len)
field of SRP_CMD requests. According to the SRP specification, the layout
of this single-byte field is as follows:
* Bits 0 and 1 are reserved.
* Bits 2 to 7 represent the ADDITIONAL CDB LENGTH field, symbolically
  represented as n.
* Still according to the SRP specification, the ADDITIONAL CDB section
  takes 4*n bytes.

Currently libsrp is only used by the ibmvscsi driver. Since the ibmvscsi
driver doesn't support large CDB's, this bug hasn't caused any problems yet.

[jejb: use & ~3 to mask the bits]
Signed-off-by: Bart Van Assche <bart.vanassche@gmail.com>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

drivers/scsi/libsrp.c

@@ -328,7 +328,7 @@ int srp_transfer_data(struct scsi_cmnd *sc, struct srp_cmd *cmd,
 	int offset, err = 0;
 	u8 format;
 
-	offset = cmd->add_cdb_len * 4;
+	offset = cmd->add_cdb_len & ~3;
 
 	dir = srp_cmd_direction(cmd);
 	if (dir == DMA_FROM_DEVICE)
@@ -366,7 +366,7 @@ static int vscsis_data_length(struct srp_cmd *cmd, enum dma_data_direction dir)
 {
 	struct srp_direct_buf *md;
 	struct srp_indirect_buf *id;
-	int len = 0, offset = cmd->add_cdb_len * 4;
+	int len = 0, offset = cmd->add_cdb_len & ~3;
 	u8 fmt;
 
 	if (dir == DMA_TO_DEVICE)