net: bridge: check port state before br_allowed_egress

If we make sure that br_allowed_egress is called only when we have
BR_STATE_FORWARDING state then we can avoid a test later when we add
per-vlan state.

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