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: 664b0f8cd8 ("netfilter: nf_tables: add generation mask to chains")
Signed-off-by: JingPiao Chen <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_tables_api.c

···
 
 		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]) {