Merge branch 'gre-fix-default-ipv6-multicast-route-creation'

Guillaume Nault says:

====================
gre: Fix default IPv6 multicast route creation.

When fixing IPv6 link-local address generation on GRE devices with
commit 3e6a0243ff00 ("gre: Fix again IPv6 link-local address
generation."), I accidentally broke the default IPv6 multicast route
creation on these GRE devices.

Fix that in patch 1, making the GRE specific code yet a bit closer to
the generic code used by most other network interface types.

Then extend the selftest in patch 2 to cover this case.
====================

Link: https://patch.msgid.link/cover.1752070620.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>