fs: new helper: simple_rename_timestamp

A rename potentially involves updating 4 different inode timestamps. Add
a function that handles the details sanely, and convert the libfs.c
callers to use it.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230705185812.579118-3-jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>