netfilter: ipset: add a coding-style fix to ip_set_ext_destroy.

Use a local variable to hold comment in order to align the arguments of
ip_set_comment_free properly.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Acked-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>