[BRIDGE]: Properly dereference the br_should_route_hook

[ Upstream commit: 82de382c ]

This hook is protected with the RCU, so simple

if (br_should_route_hook)
	br_should_route_hook(...)

is not enough on some architectures.

Use the rcu_dereference/rcu_assign_pointer in this case.

Fixed Stephen's comment concerning using the typeof().
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>