Merge branch 'ipv6-honor-oif-when-choosing-nexthop-for-locally-generated-traffic'

Ido Schimmel says:

====================
ipv6: Honor oif when choosing nexthop for locally generated traffic

Patch #1 is a preparation patch following the comment from Sashiko on
v2. See details in the commit message.

Patch #2 aligns IPv6 with IPv4 and changes IPv6 route lookup to prefer a
nexthop whose nexthop device matches the specified oif.

Patch #3 adds a selftest.
====================

Link: https://patch.msgid.link/20260611154605.992528-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>