iproute2: support listing devices by group

User can specify device group to list by using the group keyword:

	ip link show group test

If no group is specified, 0 (default) is implied.

Signed-off-by: Vlad Dogaru <ddvlad@rosedu.org>
diff --git a/ip/ipaddress.c b/ip/ipaddress.c
index a775ecd..e4748e3 100644
--- a/ip/ipaddress.c
+++ b/ip/ipaddress.c
@@ -49,6 +49,7 @@
 	char *flushb;
 	int flushp;
 	int flushe;
+	int group;
 } filter;
 
 static int do_link;
@@ -246,6 +247,12 @@
 	    fnmatch(filter.label, RTA_DATA(tb[IFLA_IFNAME]), 0))
 		return 0;
 
+	if (tb[IFLA_GROUP]) {
+		int group = *(int*)RTA_DATA(tb[IFLA_GROUP]);
+		if (group != filter.group)
+			return -1;
+	}
+
 	if (n->nlmsg_type == RTM_DELLINK)
 		fprintf(fp, "Deleted ");
 
@@ -718,9 +725,12 @@
 	if (filter.family == AF_UNSPEC)
 		filter.family = preferred_family;
 
+	filter.group = INIT_NETDEV_GROUP;
+
 	if (flush) {
 		if (argc <= 0) {
 			fprintf(stderr, "Flush requires arguments.\n");
+
 			return -1;
 		}
 		if (filter.family == AF_PACKET) {
@@ -779,6 +789,10 @@
 		} else if (strcmp(*argv, "label") == 0) {
 			NEXT_ARG();
 			filter.label = *argv;
+		} else if (strcmp(*argv, "group") == 0) {
+			NEXT_ARG();
+			if (rtnl_group_a2n(&filter.group, *argv))
+				invarg("Invalid \"group\" value\n", *argv);
 		} else {
 			if (strcmp(*argv, "dev") == 0) {
 				NEXT_ARG();