iscsi: Don't blindly use designator length in response for memcpy

Per SCSI definition the designator_length we receive from INQUIRY is 8,
12 or at most 16, but we should be careful because the remote iscsi
target may misbehave, otherwise we could have a buffer overflow.

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/iscsi.c

@@ -2226,7 +2226,7 @@ static void iscsi_populate_target_desc(unsigned char *desc, IscsiLun *lun)
     desc[5] = (dd->designator_type & 0xF)
         | ((dd->association & 3) << 4);
     desc[7] = dd->designator_length;
-    memcpy(desc + 8, dd->designator, dd->designator_length);
+    memcpy(desc + 8, dd->designator, MIN(dd->designator_length, 20));
 
     desc[28] = 0;
     desc[29] = (lun->block_size >> 16) & 0xFF;