netfilter: nf_tables: fix update chain error

 # nft add table filter
 # nft add chain filter c1
 # nft rename chain filter c1 c2

Error: Could not process rule: No such file or directory
rename chain filter c1 c2
^^^^^^^^^^^^^^^^^^^^^^^^^^

 # nft add chain filter c2
 # nft rename chain filter c1 c2
 # nft list table filter

table ip filter {
	chain c2 {
	}

	chain c2 {
	}
}

Fixes: 664b0f8c ("netfilter: nf_tables: add generation mask to chains")
Signed-off-by: JingPiao Chen <chenjingpiao@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1487,8 +1487,8 @@ static int nf_tables_updchain(struct nft_ctx *ctx, u8 genmask, u8 policy,
 
 		chain2 = nf_tables_chain_lookup(table, nla[NFTA_CHAIN_NAME],
 						genmask);
-		if (IS_ERR(chain2))
-			return PTR_ERR(chain2);
+		if (!IS_ERR(chain2))
+			return -EEXIST;
 	}
 
 	if (nla[NFTA_CHAIN_COUNTERS]) {