netns: fix error code when the nsid is already used

When the user tries to assign a specific nsid, idr_alloc() is called with
the range [nsid, nsid+1]. If this nsid is already used, idr_alloc() returns
ENOSPC (No space left on device). In our case, it's better to return
EEXIST to make it clear that the nsid is not available.

CC: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -642,6 +642,7 @@ static int rtnl_net_newid(struct sk_buff *skb, struct nlmsghdr *nlh,
 		rtnl_net_notifyid(net, RTM_NEWNSID, err);
 		err = 0;
 	} else if (err == -ENOSPC && nsid >= 0) {
+		err = -EEXIST;
 		NL_SET_BAD_ATTR(extack, tb[NETNSA_NSID]);
 		NL_SET_ERR_MSG(extack, "The specified nsid is already used");
 	}