net/ipv6/ndisc: constify ctl_table arguments of utility function

The sysctl core is preparing to only expose instances of
struct ctl_table as "const".
This will also affect the ctl_table argument of sysctl handlers.

As the function prototype of all sysctl handlers throughout the tree
needs to stay consistent that change will be done in one commit.

To reduce the size of that final commit, switch utility functions which
are not bound by "typedef proc_handler" to "const struct ctl_table".

No functional change.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20240527-sysctl-const-handler-net-v1-4-16523767d0b2@weissschuh.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>