netfilter: core: add missing __rcu annotation

removes following sparse error:
net/netfilter/core.c:598:30: warning: incorrect type in argument 1 (different address spaces)
net/netfilter/core.c:598:30:    expected struct nf_hook_entries **e
net/netfilter/core.c:598:30:    got struct nf_hook_entries [noderef] <asn:4>**<noident>

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>