netfilter: ipset: Increase the number of maximal sets automatically

The max number of sets was hardcoded at kernel cofiguration time and
could only be modified via a module parameter. The patch adds the support
of increasing the max number of sets automatically, as needed.

The array of sets is incremented by 64 new slots if we run out of
empty slots. The absolute limit for the maximal number of sets
is limited by 65534.

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