net/ipv4/netfilter: Update comment from call_rcu_bh() to call_rcu()

The RCU flavors have been consolidated, so this commit replaces a
comment's mention of call_rcu_bh() with call_rcu().

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: <netfilter-devel@vger.kernel.org>
Cc: <coreteam@netfilter.org>
Cc: <netdev@vger.kernel.org>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>