Merge branch 'ipv6-allow-for-nexthop-device-mismatch-with-onlink'

Ido Schimmel says:

====================
ipv6: Allow for nexthop device mismatch with "onlink"

This patchset aligns IPv6 with IPv4 with respect to the "onlink" keyword
and allows IPv6 routes to be configured with a gateway address that is
resolved out of a different interface than the one specified.

Patches #1-#3 are small preparations in the existing "onlink" selftest.

Patch #4 is the actual change. See the commit message for detailed
description and motivation.

Patch #5 adds test cases for both address families, to make sure that
this use case does not regress.
====================

Link: https://patch.msgid.link/20260111120813.159799-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>