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

Because opt_kfree_rcu() just calls kfree(), all call_rcu() uses of it
may be converted to kfree_rcu().  This permits 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