netfilter: ipset: Support the -exist flag with the destroy command

The -exist flag was supported with the create, add and delete commands.
In order to gracefully handle the destroy command with nonexistent sets,
the -exist flag is added to destroy too.

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