[BRIDGE]: forwarding remove unneeded preempt and bh diasables

Optimize the forwarding and transmit paths. Both places are
called with bottom half/no preempt so there is no need to use
spin_lock_bh or rcu_read_lock.

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