netfilter: nf_tables: unbind non-anonymous set if rule construction fails

Otherwise a dangling reference to a rule object that is gone remains
in the set binding list.

Fixes: 26b5a571 ("netfilter: nf_tables: add NFT_TRANS_PREPARE_ERROR to deal with bound set/chain")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>