Merge branch 'gre-revert-ipv6-link-local-address-fix'

Guillaume Nault says:

====================
gre: Revert IPv6 link-local address fix.

Following Paolo's suggestion, let's revert the IPv6 link-local address
generation fix for GRE devices. The patch introduced regressions in the
upstream CI, which are still under investigation.

Start by reverting the kselftest that depend on that fix (patch 1), then
revert the kernel code itself (patch 2).
====================

Link: https://patch.msgid.link/cover.1742418408.git.gnault@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>