target: Fix reading of data length fields for UNMAP commands

commit 1a5fa457 upstream.

The UNMAP DATA LENGTH and UNMAP BLOCK DESCRIPTOR DATA LENGTH fields
are in the unmap descriptor (the payload transferred to our data out
buffer), not in the CDB itself.  Read them from the correct place in
target_emulated_unmap.
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/target/target_core_cdb.c

@@ -1114,7 +1114,6 @@ int target_emulate_unmap(struct se_task *task)
 	struct se_cmd *cmd = task->task_se_cmd;
 	struct se_device *dev = cmd->se_dev;
 	unsigned char *buf, *ptr = NULL;
-	unsigned char *cdb = &cmd->t_task_cdb[0];
 	sector_t lba;
 	unsigned int size = cmd->data_length, range;
 	int ret = 0, offset;
@@ -1130,11 +1129,12 @@ int target_emulate_unmap(struct se_task *task)
 	/* First UNMAP block descriptor starts at 8 byte offset */
 	offset = 8;
 	size -= 8;
-	dl = get_unaligned_be16(&cdb[0]);
-	bd_dl = get_unaligned_be16(&cdb[2]);
 
 	buf = transport_kmap_data_sg(cmd);
 
+	dl = get_unaligned_be16(&buf[0]);
+	bd_dl = get_unaligned_be16(&buf[2]);
+
 	ptr = &buf[offset];
 	pr_debug("UNMAP: Sub: %s Using dl: %hu bd_dl: %hu size: %hu"
 		" ptr: %p\n", dev->transport->name, dl, bd_dl, size, ptr);