virNetDevSwitchdevFeature: Make failure to get 'family_id' non-fatal

I've just got a new machine and I'm still converging on the
kernel config. Anyway, since I don't have enabled any of SRIO-V
drivers, my kernel doesn't have NET_DEVLINK enabled (i.e.
virNetDevGetFamilyId() returns 0). But this makes nodedev driver
ignore all interfaces, because when enumerating all devices via
udev, the control reaches virNetDevSwitchdevFeature() eventually
and subsequently virNetDevGetFamilyId() which 'fails'. Well, it's
not really a failure - the virNetDevSwitchdevFeature() stub
simply returns 0.

Also, move the call a few lines below, just around the place
where it's needed.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/util/virnetdev.c

@@ -3146,9 +3146,7 @@ virNetDevSwitchdevFeature(const char *ifname,
     int is_vf = -1;
     int ret = -1;
     uint32_t family_id;
-
-    if (virNetDevGetFamilyId(DEVLINK_GENL_NAME, &family_id) <= 0)
-        return ret;
+    int rv;
 
     if ((is_vf = virNetDevIsVirtualFunction(ifname)) < 0)
         return ret;
@@ -3168,6 +3166,9 @@ virNetDevSwitchdevFeature(const char *ifname,
         goto cleanup;
     }
 
+    if ((rv = virNetDevGetFamilyId(DEVLINK_GENL_NAME, &family_id)) <= 0)
+        return rv;
+
     if (!(nl_msg = nlmsg_alloc_simple(family_id,
                                       NLM_F_REQUEST | NLM_F_ACK))) {
         virReportOOMError();