netfilter: nf_tables: free set name in error path

set->name must be free'd here in case ops->init fails.

Fixes: 38745490 ("netfilter: nf_tables: Allow set names of up to 255 chars")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>