libnetlink: don't print NETLINK_SOCK_DIAG errors in rtnl_talk

This change is a no-op, as currently no code uses rtnl_talk on
NETLINK_SOCK_DIAG_BY_FAMILY sockets. It is needed to suppress
spurious errors when using SOCK_DESTROY via rtnl_talk.
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>

lib/libnetlink.c

@@ -419,8 +419,10 @@ int rtnl_talk(struct rtnl_handle *rtnl, struct nlmsghdr *n,
 					return 0;
 				}
 
-				fprintf(stderr, "RTNETLINK answers: %s\n",
-					strerror(-err->error));
+				if (rtnl->proto != NETLINK_SOCK_DIAG)
+					fprintf(stderr,
+						"RTNETLINK answers: %s\n",
+						strerror(-err->error));
 				errno = -err->error;
 				return -1;
 			}