linux 6.7 compat: use inode atime/mtime accessors

6.6 made i_ctime inaccessible; 6.7 has done the same for i_atime and
i_mtime. This extends the method used for ctime in b37f29341 to atime
and mtime as well.

Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://github.com/sponsors/robn