if.c - vlan parent devices automatically disable

disable the child interface and change parent and enable interface after the change made...

issue reported by Tom... fix authored by stsp 
tested by Tom and reviewed OK Tom and OK Chris

if.c

@@ -2345,7 +2345,8 @@ intrtlabel(char *ifname, int ifs, int argc, char **argv)
 int
 intparent(char *ifname, int ifs, int argc, char **argv)
 {
-	int set;
+	int set, ret;
+	unsigned long cmd;
 	struct if_parent ifp;
 
 	if (NO_ARG(argv[0])) {
@@ -2374,9 +2375,22 @@ intparent(char *ifname, int ifs, int argc, char **argv)
 		return 0;
 	}
 
-	if (ioctl(ifs, set ? SIOCSIFPARENT : SIOCDIFPARENT, &ifp) == -1)
-		printf("%% intparent: SIOC%sIFPARENT: %s\n", set ? "S" : "D",
-		    strerror(errno));
+	cmd = (set ? SIOCSIFPARENT : SIOCDIFPARENT);
+	ret = ioctl(ifs, cmd, &ifp);
+	if (ret == -1) {
+	if (errno == EBUSY) {
+	                int flags = get_ifflags(ifname, ifs);
+	                  if (flags & IFF_UP) {
+                        /* Toggle interface down and retry. and bring the interface back up*/
+	                        set_ifflags(ifname, ifs, flags & ~IFF_UP);
+	                        ret = ioctl(ifs, cmd, &ifp);
+	                        set_ifflags(ifname, ifs, flags);
+	                }
+	        }
+	        if (ret == -1)
+	                printf("%% intparent: SIOC%sIFPARENT: %s\n",
+	                    set ? "S" : "D", strerror(errno));
+	}
 
 	return 0;
 }