selftests: forwarding: bridge_mdb_max: Check iproute2 version

selftests: forwarding: bridge_mdb_max: Check iproute2 version

The selftest relies on iproute2 changes present in version 6.3, but the
test does not check for it, resulting in errors:

 # ./bridge_mdb_max.sh
  INFO: 802.1d tests
  TEST: cfg4: port: ngroups reporting                                 [FAIL]
          Number of groups was null, now is null, but 5 expected
  TEST: ctl4: port: ngroups reporting                                 [FAIL]
          Number of groups was null, now is null, but 5 expected
  TEST: cfg6: port: ngroups reporting                                 [FAIL]
          Number of groups was null, now is null, but 5 expected
  [...]

Fix by skipping the test if iproute2 is too old.

Fixes: 3446dcd7df05 ("selftests: forwarding: bridge_mdb_max: Add a new selftest")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/6b04b2ba-2372-6f6b-3ac8-b7cba1cfae83@alu.unizg.hr/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-5-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>