netfilter: nf_tables: fix type in parsing in nf_tables_set_alloc_name()

In nf_tables_set_alloc_name(), we are trying to find a new, unused
name for our new set and interate through the list of present sets.
As far as I can see, we're using format string %d to parse already
present names in order to mark their presence in a bitmap, so that
we can later on find the first 0 in that map to assign the new set
name to. We should rather use a temporary variable of type int to
store the result of sscanf() to, and for making sanity checks on.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>