netfilter: nftables: extend error reporting for chain updates

The initial support for netlink extended ACK is missing the chain update
path, which results in misleading error reporting in case of EEXIST.

Fixes 36dd1bcc

 ("netfilter: nf_tables: initial support for extended ACK reporting")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>