selftests: Add IPv6 link-local address generation tests for GRE devices.

GRE devices have their special code for IPv6 link-local address
generation that has been the source of several regressions in the past.

Add selftest to check that all gre, ip6gre, gretap and ip6gretap get an
IPv6 link-link local address in accordance with the
net.ipv6.conf.<dev>.addr_gen_mode sysctl.

Note: This patch was originally applied as commit 6f50175c ("selftests:
  Add IPv6 link-local address generation tests for GRE devices.").
  However, it was then reverted by commit 355d940f ("Revert "selftests:
  Add IPv6 link-local address generation tests for GRE devices."")
  because the commit it depended on was going to be reverted. Now that
  the situation is resolved, we can add this selftest again (no changes
  since original patch, appart from context update in
  tools/testing/selftests/net/Makefile).
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/2c3a5733cb3a6e3119504361a9b9f89fda570a2d.1746225214.git.gnault@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>