[bug report] selftests: Add nettest

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello David Ahern,

The patch acda655fefae: "selftests: Add nettest" from Aug 1, 2019,
leads to the following static checker warning:

	./tools/testing/selftests/net/nettest.c:1690 main()
	warn: unsigned 'tmp' is never less than zero.

./tools/testing/selftests/net/nettest.c
  1680                  case '1':
  1681                          args.has_expected_raddr = 1;
  1682                          if (convert_addr(&args, optarg,
  1683                                           ADDR_TYPE_EXPECTED_REMOTE))
  1684                                  return 1;
  1685  
  1686                          break;
  1687                  case '2':
  1688                          if (str_to_uint(optarg, 0, 0x7ffffff, &tmp) != 0) {
  1689                                  tmp = get_ifidx(optarg);
  1690                                  if (tmp < 0) {

"tmp" is unsigned so it can't be negative.  Also all the callers assume
that get_ifidx() returns negatives on error but it looks like it really
returns zero on error so it's a bit unclear to me.

  1691                                          fprintf(stderr,
  1692                                                  "Invalid device index\n");
  1693                                          return 1;
  1694                                  }
  1695                          }
  1696                          args.expected_ifindex = (int)tmp;
  1697                          break;
  1698                  case 'q':
  1699                          quiet = 1;

regards,
dan carpenter