net: bridge: add helper to retrieve the current ageing time

The SWITCHDEV_ATTR_ID_BRIDGE_AGEING_TIME attribute is only emitted from:

sysfs/ioctl/netlink
-> br_set_ageing_time
   -> __set_ageing_time

therefore not at bridge port creation time, so:
(a) switchdev drivers have to hardcode the initial value for the address
    ageing time, because they didn't get any notification
(b) that hardcoded value can be out of sync, if the user changes the
    ageing time before enslaving the port to the bridge

We need a helper in the bridge, such that switchdev drivers can query
the current value of the bridge ageing time when they start offloading
it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>