pidns: move is-ancestor logic to helper

This check will be needed in later patches, and there's no point
open-coding it each time.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/20250805-procfs-pidns-api-v4-1-705f984940e7@cyphar.com


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