Merge branch 'gre-fix-regressions-in-ipv6-link-local-address-generation'

Guillaume Nault says:

====================
gre: Fix regressions in IPv6 link-local address generation.

IPv6 link-local address generation has some special cases for GRE
devices. This has led to several regressions in the past, and some of
them are still not fixed. This series fixes the remaining problems,
like the ipv6.conf.<dev>.addr_gen_mode sysctl being ignored and the
router discovery process not being started (see details in patch 1).

To avoid any further regressions, patch 2 adds selftests covering
IPv4 and IPv6 gre/gretap devices with all combinations of currently
supported addr_gen_mode values.
====================

Link: https://patch.msgid.link/cover.1741375285.git.gnault@redhat.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>