selftests: forwarding: bridge_mdb: Check iproute2 version

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

 # ./bridge_mdb.sh

 INFO: # Host entries configuration tests
 TEST: Common host entries configuration tests (IPv4)                [FAIL]
         Managed to add IPv4 host entry with a filter mode
 TEST: Common host entries configuration tests (IPv6)                [FAIL]
         Managed to add IPv6 host entry with a filter mode
 TEST: Common host entries configuration tests (L2)                  [FAIL]
         Managed to add L2 host entry with a filter mode

 INFO: # Port group entries configuration tests - (*, G)
 Command "replace" is unknown, try "bridge mdb help".
 [...]

Fix by skipping the test if iproute2 is too old.

Fixes: b6d00da0

 ("selftests: forwarding: Add bridge MDB test")
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/20230808141503.4060661-4-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>