linux: fix reporting NL_RTM_DELLINK to Netlink sockets

The problem is that ifname_bsd_to_linux_name() requires the interface to
exist.  But when we are in the context of ifnet_departure_event
EVENTHANDLER(9), it does not.  Instead of silently dropping the message,
let's send the FreeBSD name verbatim.  At the moment special translation
is done for IFT_LOOPBACK and IFT_ETHER only, and these two kinds of
interfaces usually don't depart.  So, this is not a final fix, but
definitely an improvement.  While here, simplify the associated code.

Differential Revision:	https://reviews.freebsd.org/D51927

sys/compat/linux/linux_netlink.c

@@ -241,25 +241,10 @@ nlmsg_copy_nla(const struct nlattr *nla_orig, struct nl_writer *nw)
 	return (false);
 }
 
-/*
- * Translate a FreeBSD interface name to a Linux interface name.
- */
-static bool
-nlmsg_translate_ifname_nla(struct nlattr *nla, struct nl_writer *nw)
-{
-	char ifname[LINUX_IFNAMSIZ];
-
-	if (ifname_bsd_to_linux_name((char *)(nla + 1), ifname,
-	    sizeof(ifname)) <= 0)
-		return (false);
-	return (nlattr_add_string(nw, IFLA_IFNAME, ifname));
-}
-
-#define	LINUX_NLA_UNHANDLED	-1
 /*
  * Translate a FreeBSD attribute to a Linux attribute.
- * Returns LINUX_NLA_UNHANDLED when the attribute is not processed
- * and the caller must take care of it, otherwise the result is returned.
+ * Returns false when the attribute is not processed and the caller must take
+ * care of it.
  */
 static int
 nlmsg_translate_all_nla(struct nlmsghdr *hdr, struct nlattr *nla,
@@ -271,22 +256,27 @@ nlmsg_translate_all_nla(struct nlmsghdr *hdr, struct nlattr *nla,
 	case NL_RTM_DELLINK:
 	case NL_RTM_GETLINK:
 		switch (nla->nla_type) {
-		case IFLA_IFNAME:
-			return (nlmsg_translate_ifname_nla(nla, nw));
+		case IFLA_IFNAME: {
+			char ifname[LINUX_IFNAMSIZ];
+
+			if (ifname_bsd_to_linux_name((char *)(nla + 1), ifname,
+			    sizeof(ifname)) > 0)
+				return (true);
+			break;
+		}
 		default:
 			break;
 		}
 	default:
 		break;
 	}
-	return (LINUX_NLA_UNHANDLED);
+	return (false);
 }
 
 static bool
 nlmsg_copy_all_nla(struct nlmsghdr *hdr, int raw_hdrlen, struct nl_writer *nw)
 {
 	struct nlattr *nla;
-	int ret;
 
 	int hdrlen = NETLINK_ALIGN(raw_hdrlen);
 	int attrs_len = hdr->nlmsg_len - sizeof(struct nlmsghdr) - hdrlen;
@@ -297,15 +287,12 @@ nlmsg_copy_all_nla(struct nlmsghdr *hdr, int raw_hdrlen, struct nl_writer *nw)
 		if (nla->nla_len < sizeof(struct nlattr)) {
 			return (false);
 		}
-		ret = nlmsg_translate_all_nla(hdr, nla, nw);
-		if (ret == LINUX_NLA_UNHANDLED)
-			ret = nlmsg_copy_nla(nla, nw);
-		if (!ret)
+		if (!nlmsg_translate_all_nla(hdr, nla, nw) &&
+		    !nlmsg_copy_nla(nla, nw))
 			return (false);
 	}
 	return (true);
 }
-#undef LINUX_NLA_UNHANDLED
 
 static unsigned int
 rtnl_if_flags_to_linux(unsigned int if_flags)