libfs: export find_next_child()

Export find_next_child() so it can be used by efivarfs.
Keep it internal for now. There's no reason to advertise this
kernel-wide.

Link: https://lore.kernel.org/r/20250331-work-freeze-v1-1-6dfbe8253b9f@kernel.org


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