bridge: netlink: export port's timer values

Add the following attributes in order to export port's timer values:
IFLA_BRPORT_MESSAGE_AGE_TIMER, IFLA_BRPORT_FORWARD_DELAY_TIMER and
IFLA_BRPORT_HOLD_TIMER.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>