netfilter: nf_tables: fix dumping with large number of sets

If not table name is specified, the dumping of the existing sets
may be incomplete with a sufficiently large number of sets and
tables. This patch fixes missing reset of the cursors after
finding the location of the last object that has been included
in the previous multi-part message.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>