scripts: check-sysctl-docs: handle per-namespace sysctls

Some sysctl tables are registered for each namespace.
(Like in ipc/ipc_sysctl.c)
These need special handling to track the variable assignments.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Joel Granados <j.granados@samsung.com>