net: dsa: netc: implement dynamic FDB entry ageing

net: dsa: netc: implement dynamic FDB entry ageing

The NETC switch does not age out dynamic FDB entries automatically.
Without software management, stale entries persist after topology
changes and cause incorrect forwarding.

Add a delayed work that periodically removes entries that have not been
refreshed within the specified cycles. The effective ageing time is:

  ageing_time = fdbt_ageing_delay * 100

Default values are 3s interval and 100 cycles (300s total), matching
the IEEE 802.1Q default ageing time. The work starts when the first
port joins a bridge (tracked via br_cnt) and is cancelled when the
last port leaves. All FDB operations are serialized under fdbt_lock.

Implement .set_ageing_time() to allow the bridge layer to reconfigure
ageing parameters on demand.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20260611021458.2629145-10-wei.fang@oss.nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>