netfilter: nf_tables: really skip inactive sets when allocating name

While looping to build the bitmap of used anonymous set names, check the
current set in the iteration, instead of the one that is being created.

Fixes: 37a9cc52 ("netfilter: nf_tables: add generation mask to sets")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>