ipv6 addrconf: remove addrconf_sysctl_hop_limit()

This is an effective no-op in terms of user observable behaviour.

By preventing the overwrite of non-null extra1/extra2 fields
in addrconf_sysctl() we can enable the use of proc_dointvec_minmax().

This allows us to eliminate the constant min/max (1..255) trampoline
function that is addrconf_sysctl_hop_limit().

This is nice because it simplifies the code, and allows future
sysctls with constant min/max limits to also not require trampolines.

We still can't eliminate the trampoline for mtu because it isn't
actually a constant (it depends on other tunables of the device)
and thus requires at-write-time logic to enforce range.

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Acked-by: Erik Kline <ek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>