devlink: fix potential memort leak

We must free allocated skb when genlmsg_put() return fails.

Fixes: 1555d204 ("devlink: Support for pipeline debug (dpipe)")
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>