ipv4: Convert call_rcu() to kfree_rcu(), drop opt_kfree_rcu

The call_rcu() in do_ip_setsockopt() invokes opt_kfree_rcu(), which just
calls kfree().  So convert the call_rcu() to kfree_rcu(), which allows
opt_kfree_rcu() to be eliminated.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: James Morris <jmorris@namei.org>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Patrick McHardy <kaber@trash.net>
Cc: netdev@vger.kernel.org