sysctl: move internal interfaces to const struct ctl_table

As a preparation to make all the core sysctl code work with const struct
ctl_table switch over the internal function to use the const variant.

Some pointers to "struct ctl_table" need to stay non-const as they are
newly allocated and modified before registration.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Joel Granados <joel.granados@kernel.org>