selftests: net: forwarding: fix IPv6 address leak in cleanup

selftests: net: forwarding: fix IPv6 address leak in cleanup

Several forwarding tests (e.g., gre_multipath.sh) initialize both IPv4
and IPv6 addresses using simple_if_init, but only clean up IPv4
in simple_if_fini. This leaves stale IPv6 addresses on the interfaces,
which causes subsequent tests to fail when they encounter unexpected
address configuration.

The issue can be reproduced by running tests in sequence:
  # run_kselftest.sh -t net/forwarding:ipip_hier_gre.sh
  # run_kselftest.sh -t net/forwarding:min_max_mtu.sh
  TAP version 13
  1..1
  # timeout set to 0
  # selftests: net/forwarding: min_max_mtu.sh
  # TEST: ping                                                          [ OK ]
  # TEST: ping6                                                         [ OK ]
  # TEST: Test maximum MTU configuration                                [ OK ]
  # TEST: Test traffic, packet size is maximum MTU                      [FAIL]
  #       Ping6, packet size: 65487 succeeded, but should have failed
  # TEST: Test minimum MTU configuration                                [ OK ]
  # TEST: Test traffic, packet size is minimum MTU                      [ OK ]
  not ok 1 selftests: net/forwarding: min_max_mtu.sh # exit=1

Fix this by removing the unused IPv6 argument from simple_if_init in
tests that don't use IPv6 (gre_multipath.sh, ipip_lib.sh), and by
adding the missing IPv6 argument to simple_if_fini in tests that
use IPv6 (gre_multipath_nh.sh, gre_multipath_nh_res.sh).

Signed-off-by: Aleksei Oladko <aleksey.oladko@virtuozzo.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260305211000.515301-1-aleksey.oladko@virtuozzo.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>