luci-mod-admin-full: limit dns cachesize to 10000

The value of cachesize is hardcoded to 10000 in
dnsmasq-2.79/src/option.c to 10000 max

    case 'c':  /* --cache-size */
      {
        int size;

        if (!atoi_check(arg, &size))
          ret_err(gen_err);
        else
          {
            /* zero is OK, and means no caching. */

            if (size < 0)
              size = 0;
            else if (size > 10000)
              size = 10000;

            daemon->cachesize = size;
          }
        break;
      }

Tested on Netgear R7800
Signed-off-by: Marc Benoit <marcb62185@gmail.com>