netfilter: nf_tables: Release memory obtained by kasprintf

Free memory region, if nf_tables_set_alloc_name is not successful.

Fixes: 38745490 ("netfilter: nf_tables: Allow set names of up to 255 chars")
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>