devlink: protect devlink dump by the instance lock

Take the instance lock around devlink_nl_fill() when dumping,
doit takes it already.

We are only dumping basic info so in the worst case we were risking
data races around the reload statistics. Until the big devlink mutex
was removed all relevant code was protected by it, so the missing
instance lock was not exposed.

Fixes: d3efc2a6 ("net: devlink: remove devlink_mutex")
Reviewed-by: NJiri Pirko <jiri@nvidia.com>
Reviewed-by: NJacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20221216044122.1863550-1-kuba@kernel.orgSigned-off-by: NJakub Kicinski <kuba@kernel.org>

net/core/devlink.c

@@ -1648,10 +1648,13 @@ static int devlink_nl_cmd_get_dumpit(struct sk_buff *msg,
 			continue;
 		}
 
+		devl_lock(devlink);
 		err = devlink_nl_fill(msg, devlink, DEVLINK_CMD_NEW,
 				      NETLINK_CB(cb->skb).portid,
 				      cb->nlh->nlmsg_seq, NLM_F_MULTI);
+		devl_unlock(devlink);
 		devlink_put(devlink);
+
 		if (err)
 			goto out;
 		idx++;