qcow2: mark this file's sole strncpy use as justified

Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

block/qcow2.c

@@ -1096,6 +1096,7 @@ int qcow2_update_header(BlockDriverState *bs)
             goto fail;
         }
 
+        /* Using strncpy is ok here, since buf is not NUL-terminated. */
         strncpy(buf, bs->backing_file, buflen);
 
         header->backing_file_offset = cpu_to_be64(buf - ((char*) header));