Add a new must_set_pos() helper. We will use it in follow-up patches.
Temporarily mark it as unused. This is only done to keep the diff small
and reviewable.
Link: https://lore.kernel.org/r/20240830-vfs-file-f_version-v1-6-6d3e4816aa7b@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>