netlink: Fix IFF_UP flag handling in RTM_NEWLINK's modify_link handler

IFF_UP could previously only be unset via RTM_NEWLINK. Requests to set
IFF_UP, though they succeed, did not actually set the flag.

RTM_NEWLINK messages with ifi_change=0 are treated as
ifi_change=0xFFFFFFFF, modifying all the link flags (currently IFF_UP
and IFF_PROMISC) to match the behavior seen on Linux.

Reviewed by:	obiwac, kp, mckusick (mentor)
Approved by:	obiwac, mckusick (mentor)
Sponsored by:	Google LLC (GSoC)
Differential Revision:	https://reviews.freebsd.org/D51871

sys/netlink/route/iface_drivers.c

@@ -82,9 +82,12 @@ _nl_modify_ifp_generic(struct ifnet *ifp, struct nl_parsed_link *lattrs,
 		}
 	}
 
-	if ((lattrs->ifi_change & IFF_UP) && (lattrs->ifi_flags & IFF_UP) == 0) {
-		/* Request to down the interface */
-		if_down(ifp);
+	if ((lattrs->ifi_change & IFF_UP) != 0 || lattrs->ifi_change == 0) {
+		/* Request to up or down the interface */
+		if (lattrs->ifi_flags & IFF_UP)
+			if_up(ifp);
+		else
+			if_down(ifp);
 	}
 
 	if (lattrs->ifla_mtu > 0) {
@@ -97,7 +100,8 @@ _nl_modify_ifp_generic(struct ifnet *ifp, struct nl_parsed_link *lattrs,
 		}
 	}
 
-	if (lattrs->ifi_change & IFF_PROMISC) {
+	if ((lattrs->ifi_change & IFF_PROMISC) != 0 ||
+	    lattrs->ifi_change == 0) {
 		error = ifpromisc(ifp, lattrs->ifi_flags & IFF_PROMISC);
 		if (error != 0) {
 			nlmsg_report_err_msg(npt, "unable to set promisc");