netfilter: ebtables: make broute table work again

broute table init hook sets up the "br_should_route_hook" pointer,
which then gets called from br_input.

commit a386f990
(bridge: add proper RCU annotation to should_route_hook)
introduced a typedef, and then changed this to:

br_should_route_hook_t *rhook;
[..]
rhook = rcu_dereference(br_should_route_hook);
if (*rhook(skb))

problem is that "br_should_route_hook" contains the address of the function,
so calling *rhook() results in kernel panic.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>