ipset: fix json output format for IPSET_OPT_IP

ipset: fix json output format for IPSET_OPT_IP

IPSET_OPT_IP should be quoted to be a well formed json file, otherwise see
following bad example (range is not quoted):

  # ipset create foo bitmap:ip range 192.168.0.0/16
  # ipset list -o json foo
  [
    {
      "name" : "foo",
      "type" : "bitmap:ip",
      "revision" : 3,
      "header" : {
        "range" : 192.168.0.0-192.168.255.255,
        "memsize" : 8280,
        "references" : 0,
        "numentries" : 0
      },
      "members" : [
      ]
    }
  ]

Signed-off-by: Z. Liu <liuzx@knownsec.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>