fs: new accessor methods for atime and mtime

Recently, we converted the ctime accesses in the kernel to use new
accessor functions. Linus recently pointed out though that if we add
accessors for the atime and mtime, then that would allow us to
seamlessly change how these timestamps are stored in the inode.

Add new accessor functions for the atime and mtime that mirror the
accessors for the ctime.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20231004185239.80830-1-jlayton@kernel.org


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