rtnetlink: let rtnl_bridge_setlink check IFLA_BRIDGE_MODE length

JIRA: https://issues.redhat.com/browse/RHEL-18148

Tested: With affected drivers (ice, i40e) and drivers where no behavior change is expected (bnxt_en, mlx5_core). Reproducer is attached in Jira.

Signed-off-by: Michal Schmidt mschmidt@redhat.com