ipv6: fix neighbour resolution with raw socket

[Nicolas Dichtel]

Hi Ubuntu kernel team,

please, can you consider to cherry-pick the following linux upstream fixes:
9b1c1ef13b35 ("ipv6: constify rt6_nexthop()")
2c6b55f45d53 ("ipv6: fix neighbour resolution with raw socket")

They have been included in v5.2. The patch that introduces the bug has not been
identified, but the bug is here since a long time (at least v4.4).

Here is the launchpad entry:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1834465


Thank you,
Nicolas