genetlink: fix genl_set_err() group ID

Fix another really stupid bug - I introduced genl_set_err()
precisely to be able to adjust the group and reject invalid
ones, but then forgot to do so.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/genetlink.h

@@ -384,6 +384,9 @@ static inline struct sk_buff *genlmsg_new(size_t payload, gfp_t flags)
 static inline int genl_set_err(struct genl_family *family, struct net *net,
 			       u32 portid, u32 group, int code)
 {
+	if (WARN_ON_ONCE(group >= family->n_mcgrps))
+		return -EINVAL;
+	group = family->mcgrp_offset + group;
 	return netlink_set_err(net->genl_sock, portid, group, code);
 }