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>