netlink: implement unbind to netlink_setsockopt NETLINK_DROP_MEMBERSHIP

Call the per-protocol unbind function rather than bind function on
NETLINK_DROP_MEMBERSHIP in netlink_setsockopt().

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 7e8d229..92f4b69 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2129,7 +2129,7 @@
 			return err;
 		if (!val || val - 1 >= nlk->ngroups)
 			return -EINVAL;
-		if (nlk->netlink_bind) {
+		if (optname == NETLINK_ADD_MEMBERSHIP && nlk->netlink_bind) {
 			err = nlk->netlink_bind(val);
 			if (err)
 				return err;
@@ -2138,6 +2138,8 @@
 		netlink_update_socket_mc(nlk, val,
 					 optname == NETLINK_ADD_MEMBERSHIP);
 		netlink_table_ungrab();
+		if (optname == NETLINK_DROP_MEMBERSHIP && nlk->netlink_unbind)
+			nlk->netlink_unbind(val);
 
 		err = 0;
 		break;