yaffs: fix mtime/itime field access

Adapting our inode field access to the following upstream
commit:

  commit 12cd4402


  Author: Jeff Layton <jlayton@kernel.org>
  Date:   Fri Sep 29 09:05:52 2023 -0400

    fs: rename inode i_atime and i_mtime fields

    Rename these two fields to discourage direct access (and to help ensure
    that we mop up any leftover direct accesses).

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>