bridge: split rcu and no-rcu cases of fdb lookup

In some cases, look up of forward database entry is done with RCU;
and for others no RCU is needed because of locking. Split the two
cases into two differnt loops (and take off inline).

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>