Merge branch 'gre-reapply-ipv6-link-local-address-generation-fix'

Guillaume Nault says:

====================
gre: Reapply IPv6 link-local address generation fix.

Reintroduce the IPv6 link-local address generation fix for GRE and its
kernel selftest. These patches were introduced by merge commit
b3fc5927de4b ("Merge branch
'gre-fix-regressions-in-ipv6-link-local-address-generation'") but have
been reverted by commit 8417db0be5bb ("Merge branch
'gre-revert-ipv6-link-local-address-fix'"), because it uncovered
another bug in multipath routing. Now that this bug has been
investigated and fixed, we can apply the GRE link-local address fix
and its kernel selftest again.

For convenience, here's the original cover letter:

    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.1746225213.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>