selftests: Fix get_ifidx and callers in nettest.c

Dan reported:

    The patch acda655f: "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.

Update get_ifidx to return -1 on errors and cleanup callers of it.

Fixes: acda655f ("selftests: Add nettest")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>