platform: enable parsing tun/tap properties from netlink

Now that the kernel patches are merged to mainline (rc), enable accepting
tun/tap link properties from netlink.

https://bugzilla.redhat.com/show_bug.cgi?id=1547213

src/platform/nm-linux-platform.c

@@ -1396,10 +1396,6 @@ _parse_lnk_tun (const char *kind, struct nlattr *info_data)
 	NMPObject *obj;
 	NMPlatformLnkTun *props;
 
-	/* FIXME: the netlink API is not yet part of a released kernel version
-	 *        Disable it for now, until we are sure it is stable. */
-	return NULL;
-
 	if (!info_data || !nm_streq0 (kind, "tun"))
 		return NULL;