ipv6: constify rt6_nexthop()

ipv6: constify rt6_nexthop()

BugLink: https://bugs.launchpad.net/bugs/1834465
There is no functional change in this patch, it only prepares the next one.

rt6_nexthop() will be used by ip6_dst_lookup_neigh(), which uses const
variables.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(backported from commit 9b1c1ef13b35fa35051b635ca9fbda39fe6bbc70)
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>