fix some cases of rc == 0 on error

When genl_socket_talk() fails, return rc != 0.

While testing the new osmo-upf program, I noticed that I failed to get a
"Operation not permitted" error when forgetting to set cap_net_admin on
the osmo-upf binary. It looked like everything should work, but doesn't.

It is possible to catch these error cases without this patch, by
monitoring errno. That may well be the intention of the API? I'm still
submitting this patch because it seems better to return rc != 0.

Here is a code example that also catches all error cases without this
patch:

        errno = 0;
        rc = gtp_add_tunnel(genl_id, nl, t);
        if (errno) {
                rc = -errno;
        } else if (rc) {
                rc = -EINVAL;
        } else {
                tun->active = true;
        }
	return rc;

Related: SYS#5599
Change-Id: I22fd69709e023572c6c616a4184340a554456faf
---
M src/gtp-genl.c
1 file changed, 7 insertions(+), 3 deletions(-)